diff --git a/.travis.yml b/.travis.yml
index a585d16..0af7ea6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
+sudo: false
 
 language: node_js
 
@@ -7,10 +8,18 @@ node_js:
   - 8
   - 6
 
-sudo: false
+matrix:
+  include:
+    - node_js: node
+      env: BROTLI=1
+    - node_js: 6
+      env: BROTLI=1
+      before_install: npm i --save-only request brotli
 
+before_install: npm i --save-only request
+install: npm i
 after_success: npm run coverage
 
 notifications:
     webhooks: https://www.travisbuddy.com/?insertMode=update
-    on_success: never
\ No newline at end of file
+    on_success: never
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1e178c2..a0aaf7b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 ## Change Log
 
+### v4.0.0 (22/04/2019)
+- Randomize `User-Agent` header with random chrome browser
+- Recaptcha solving support
+- Brotli non-mandatory support
+- Various code changes and improvements
+
 ### v3.9.1 (11/04/2019)
 - Fix for the timeout parsing
 
@@ -11,7 +17,7 @@
 
 ### v3.7.0 (07/04/2019)
 - [#182](https://github.com/codemanki/cloudscraper/pull/182) Usage examples have been added.
-- [#169](https://github.com/codemanki/cloudscraper/pull/169) Cloudscraper now automatically parses out timeout for a CF challenge. `cloudflareTimeout` still can be used, but will be deprecated soon
+- [#169](https://github.com/codemanki/cloudscraper/pull/169) Cloudscraper now automatically parses out timeout for a CF challenge.
 
 ### v3.6.0 (03/04/2019)
 - [#180](https://github.com/codemanki/cloudscraper/pull/180) Update code to parse latest CF challenge
diff --git a/README.md b/README.md
index 85c067c..4cfd468 100644
--- a/README.md
+++ b/README.md
@@ -127,6 +127,10 @@ Cloudscraper wraps request and request-promise, so using cloudscraper is pretty
     .catch(function (err) {
     });
   ```
+
+## Recaptcha
+Cloudscraper may help you with the recaptcha page. Take a look at [this example](https://github.com/codemanki/cloudscraper/blob/master/examples/solve-recaptcha.js).
+
 ## Defaults method
 
 `cloudscraper.defaults` is a very convenient way of extending the cloudscraper requests with any of your settings.
@@ -151,12 +155,16 @@ var options = {
   jar: requestModule.jar(), // Custom cookie jar
   headers: {
     // User agent, Cache Control and Accept headers are required
+    // User agent is populated by a random UA.
     'User-Agent': 'Ubuntu Chromium/34.0.1847.116 Chrome/34.0.1847.116 Safari/537.36',
     'Cache-Control': 'private',
     'Accept': 'application/xml,application/xhtml+xml,text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5'
   },
-  // Cloudflare requires a delay of 4 seconds, so wait for at least 5.
+  // Cloudscraper automatically parses out timeout required by Cloudflare.
+  // Override cloudflareTimeout to adjust it.
   cloudflareTimeout: 5000,
+  // Reduce Cloudflare's timeout to cloudflareMaxTimeout if it is excessive
+  cloudflareMaxTimeout: 30000,
   // followAllRedirects - follow non-GET HTTP 3xx responses as redirects
   followAllRedirects: true,
   // Support only this max challenges in row. If CF returns more, throw an error
@@ -227,3 +235,4 @@ Current Cloudflare implementation requires browser to respect the timeout of 5 s
 * [request-promise](https://github.com/request/request-promise)
 
 
+
diff --git a/errors.js b/errors.js
index f2975bf..fee304e 100644
--- a/errors.js
+++ b/errors.js
@@ -9,17 +9,17 @@
 // 1. There is a non-enumerable errorType attribute.
 // 2. The error constructor is hidden from the stacktrace.
 
-var EOL = require('os').EOL;
-var original = require('request-promise-core/errors');
-var http = require('http');
+const EOL = require('os').EOL;
+const original = require('request-promise-core/errors');
+const http = require('http');
 
-var BUG_REPORT = format([
+const BUG_REPORT = format([
   '### Cloudflare may have changed their technique, or there may be a bug.',
   '### Bug Reports: https://github.com/codemanki/cloudscraper/issues',
   '### Check the detailed exception message that follows for the cause.'
 ]);
 
-var ERROR_CODES = {
+const ERROR_CODES = {
   // Non-standard 5xx server error HTTP status codes
   '520': 'Web server is returning an unknown error',
   '521': 'Web server is down',
@@ -48,22 +48,22 @@ ERROR_CODES[1006] =
     ERROR_CODES[1007] =
         ERROR_CODES[1008] = 'Access Denied: Your IP address has been banned';
 
-var OriginalError = original.RequestError;
+const OriginalError = original.RequestError;
 
-var RequestError = create('RequestError', 0);
-var CaptchaError = create('CaptchaError', 1);
+const RequestError = create('RequestError', 0);
+const CaptchaError = create('CaptchaError', 1);
 
 // errorType 4 is a CloudflareError so this constructor is reused.
-var CloudflareError = create('CloudflareError', 2, function (error) {
+const CloudflareError = create('CloudflareError', 2, function (error) {
   if (!isNaN(error.cause)) {
-    var description = ERROR_CODES[error.cause] || http.STATUS_CODES[error.cause];
+    const description = ERROR_CODES[error.cause] || http.STATUS_CODES[error.cause];
     if (description) {
       error.message = error.cause + ', ' + description;
     }
   }
 });
 
-var ParserError = create('ParserError', 3, function (error) {
+const ParserError = create('ParserError', 3, function (error) {
   error.message = BUG_REPORT + error.message;
 });
 
diff --git a/examples/solve-recaptcha.js b/examples/solve-recaptcha.js
new file mode 100644
index 0000000..edb1094
--- /dev/null
+++ b/examples/solve-recaptcha.js
@@ -0,0 +1,19 @@
+#!/usr/bin/env node
+
+function solveReCAPTCHA (url, sitekey, callback) {
+  // Here you do some magic with the sitekey provided by cloudscraper
+}
+
+function onCaptcha (options, response, body) {
+  const captcha = response.captcha;
+  // solveReCAPTCHA is a method that you should come up with and pass it href and sitekey, in return it will return you a reponse
+  solveReCAPTCHA(response.request.uri.href, captcha.siteKey, (error, gRes) => {
+    if (error) return void captcha.submit(error);
+    captcha.form['g-recaptcha-response'] = gRes;
+    captcha.submit();
+  });
+}
+
+const cloudscraper = require('..').defaults({ onCaptcha });
+var uri = process.argv[2];
+cloudscraper.get({ uri: uri, headers: { cookie: 'captcha=1' } }).catch(console.warn).then(console.log); // eslint-disable-line promise/catch-or-return
diff --git a/index.js b/index.js
index 0201c4f..ab7bb5c 100644
--- a/index.js
+++ b/index.js
@@ -1,50 +1,31 @@
 'use strict';
 
-var vm = require('vm');
-var requestModule = require('request-promise');
-var errors = require('./errors');
-var decodeEmails = require('./lib/email-decode.js');
-
-var USER_AGENTS = [
-  'Ubuntu Chromium/34.0.1847.116 Chrome/34.0.1847.116 Safari/537.36',
-  'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.21 (KHTML, like Gecko) konqueror/4.14.10 Safari/537.21',
-  'Mozilla/5.0 (iPad; CPU OS 5_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko ) Version/5.1 Mobile/9B176 Safari/7534.48.3',
-  'Mozilla/5.0 (iPad; U; CPU OS 3_2 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B334b Safari/531.21.10',
-  'Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; Media Center PC 6.0; InfoPath.3; MS-RTC LM 8; Zune 4.7)',
-  'Mozilla/5.0 (Windows Phone 8.1; ARM; Trident/7.0; Touch; rv:11.0; IEMobile/11.0; NOKIA; Lumia 630) like Gecko',
-  'Mozilla/5.0 (compatible; MSIE 10.0; Windows Phone 8.0; Trident/6.0; IEMobile/10.0; ARM; Touch; NOKIA; Lumia 920)',
-  'Mozilla/5.0 (Linux; U; Android 2.2; en-us; Sprint APA9292KT Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1',
-  'Mozilla/5.0 (X11; Linux x86_64; rv:2.2a1pre) Gecko/20100101 Firefox/4.2a1pre',
-  'Mozilla/5.0 (SymbianOS/9.1; U; en-us) AppleWebKit/413 (KHTML, like Gecko) Safari/413 es65',
-  'Mozilla/5.0 (Linux; Android 6.0; Nexus 5X Build/MDB08L) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.124 Mobile Safari/537.36',
-  'Mozilla/5.0 (X11; U; FreeBSD i386; de-CH; rv:1.9.2.8) Gecko/20100729 Firefox/3.6.8'
-];
-
-var DEFAULT_USER_AGENT = randomUA();
-
-var VM_OPTIONS = {
-  contextOrigin: 'cloudflare:challenge.js',
-  contextCodeGeneration: { strings: true, wasm: false },
-  timeout: 5000
-};
+const requestModule = require('request-promise');
+const sandbox = require('./lib/sandbox');
+const decodeEmails = require('./lib/email-decode.js');
+const getDefaultHeaders = require('./lib/headers');
+const brotli = require('./lib/brotli');
+
+const {
+  RequestError,
+  CaptchaError,
+  CloudflareError,
+  ParserError
+} = require('./errors');
+
+const HOST = Symbol('host');
 
 module.exports = defaults.call(requestModule);
 
 function defaults (params) {
   // isCloudScraper === !isRequestModule
-  var isRequestModule = this === requestModule;
+  const isRequestModule = this === requestModule;
 
-  var defaultParams = (!isRequestModule && this.defaultParams) || {
+  let defaultParams = (!isRequestModule && this.defaultParams) || {
     requester: requestModule,
     // Cookies should be enabled
     jar: requestModule.jar(),
-    headers: {
-      'Connection': 'keep-alive',
-      'User-Agent': DEFAULT_USER_AGENT,
-      'Cache-Control': 'private',
-      'Accept': 'application/xml,application/xhtml+xml,text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5',
-      'Accept-Language': 'en-US,en;q=0.9'
-    },
+    headers: getDefaultHeaders({ 'Host': HOST }),
     // Reduce Cloudflare's timeout to cloudflareMaxTimeout if it is excessive
     cloudflareMaxTimeout: 30000,
     // followAllRedirects - follow non-GET HTTP 3xx responses as redirects
@@ -52,14 +33,17 @@ function defaults (params) {
     // Support only this max challenges in row. If CF returns more, throw an error
     challengesToSolve: 3,
     // Remove Cloudflare's email protection
-    decodeEmails: false
+    decodeEmails: false,
+    // Support gzip encoded responses
+    gzip: true
   };
 
   // Object.assign requires at least nodejs v4, request only test/supports v6+
   defaultParams = Object.assign({}, defaultParams, params);
 
-  var cloudscraper = requestModule.defaults
+  const cloudscraper = requestModule.defaults
     .call(this, defaultParams, function (options) {
+      validateRequest(options);
       return performRequest(options, true);
     });
 
@@ -79,9 +63,7 @@ function defaults (params) {
   return cloudscraper;
 }
 
-// This function is wrapped to ensure that we get new options on first call.
-// The options object is reused in subsequent calls when calling it directly.
-function performRequest (options, isFirstRequest) {
+function validateRequest (options) {
   // Prevent overwriting realEncoding in subsequent calls
   if (!('realEncoding' in options)) {
     // Can't just do the normal options.encoding || 'utf8'
@@ -105,15 +87,26 @@ function performRequest (options, isFirstRequest) {
       'got ' + typeof (options.cloudflareMaxTimeout) + ' instead.');
   }
 
-  // This should be the default export of either request or request-promise.
-  var requester = options.requester;
-
-  if (typeof requester !== 'function') {
+  if (typeof options.requester !== 'function') {
     throw new TypeError('Expected `requester` option to be a function, got ' +
-        typeof (requester) + ' instead.');
+      typeof (options.requester) + ' instead.');
   }
+}
 
-  var request = requester(options);
+// This function is wrapped to ensure that we get new options on first call.
+// The options object is reused in subsequent calls when calling it directly.
+function performRequest (options, isFirstRequest) {
+  // This should be the default export of either request or request-promise.
+  const requester = options.requester;
+
+  // Note that request is always an instanceof ReadableStream, EventEmitter
+  // If the requester is request-promise, it is also thenable.
+  const request = requester(options);
+
+  // We must define the host header ourselves to preserve case and order.
+  if (request.getHeader('host') === HOST) {
+    request.setHeader('host', request.uri.host);
+  }
 
   // If the requester is not request-promise, ensure we get a callback.
   if (typeof request.callback !== 'function') {
@@ -139,7 +132,7 @@ function performRequest (options, isFirstRequest) {
       onRequestResponse(options, null, response, body);
     });
 
-  // Indicate that this is a cloudscraper request, required by test/helper.
+  // Indicate that this is a cloudscraper request
   request.cloudscraper = true;
   return request;
 }
@@ -147,12 +140,12 @@ function performRequest (options, isFirstRequest) {
 // The argument convention is options first where possible, options
 // always before response, and body always after response.
 function onRequestResponse (options, error, response, body) {
-  var callback = options.callback;
+  const callback = options.callback;
 
   // Encoding is null so body should be a buffer object
   if (error || !body || !body.toString) {
     // Pure request error (bad connection, wrong url, etc)
-    return callback(new errors.RequestError(error, options, response));
+    return callback(new RequestError(error, options, response));
   }
 
   response.responseStartTime = Date.now();
@@ -164,23 +157,33 @@ function onRequestResponse (options, error, response, body) {
     return callback(null, response, body);
   }
 
+  // Decompress brotli compressed responses
+  if (/\bbr\b/i.test('' + response.caseless.get('content-encoding'))) {
+    if (!brotli.isAvailable) {
+      const cause = 'Received a Brotli compressed response. Please install brotli';
+      return callback(new RequestError(cause, options, response));
+    }
+
+    response.body = body = brotli.decompress(body);
+  }
+
   if (response.isCloudflare && response.isHTML) {
     onCloudflareResponse(options, response, body);
   } else {
-    processResponseBody(options, response, body);
+    onRequestComplete(options, response, body);
   }
 }
 
 function onCloudflareResponse (options, response, body) {
-  var callback = options.callback;
+  const callback = options.callback;
 
-  var stringBody;
-  var isChallenge;
-  var isRedirectChallenge;
+  let stringBody;
+  let isChallenge;
+  let isRedirectChallenge;
 
   if (body.length < 1) {
     // This is a 4xx-5xx Cloudflare response with an empty body.
-    return callback(new errors.CloudflareError(response.statusCode, options, response));
+    return callback(new CloudflareError(response.statusCode, options, response));
   }
 
   stringBody = body.toString('utf8');
@@ -188,104 +191,106 @@ function onCloudflareResponse (options, response, body) {
   try {
     validate(options, response, stringBody);
   } catch (error) {
+    if (error instanceof CaptchaError && typeof options.onCaptcha === 'function') {
+      // Give users a chance to solve the reCAPTCHA via services such as anti-captcha.com
+      return onCaptcha(options, response, stringBody);
+    }
+
     return callback(error);
   }
 
   isChallenge = stringBody.indexOf('a = document.getElementById(\'jschl-answer\');') !== -1;
 
   if (isChallenge) {
-    return solveChallenge(options, response, stringBody);
+    return onChallenge(options, response, stringBody);
   }
 
   isRedirectChallenge = stringBody.indexOf('You are being redirected') !== -1 ||
     stringBody.indexOf('sucuri_cloudproxy_js') !== -1;
 
   if (isRedirectChallenge) {
-    return setCookieAndReload(options, response, stringBody);
+    return onRedirectChallenge(options, response, stringBody);
   }
 
   // 503 status is always a challenge
   if (response.statusCode === 503) {
-    return solveChallenge(options, response, stringBody);
+    return onChallenge(options, response, stringBody);
   }
 
   // All is good
-  processResponseBody(options, response, body);
+  onRequestComplete(options, response, body);
 }
 
 function validate (options, response, body) {
-  var match;
+  let match;
 
   // Finding captcha
   if (body.indexOf('why_captcha') !== -1 || /cdn-cgi\/l\/chk_captcha/i.test(body)) {
-    throw new errors.CaptchaError('captcha', options, response);
+    // Convenience boolean
+    response.isCaptcha = true;
+    throw new CaptchaError('captcha', options, response);
   }
 
   // Trying to find '<span class="cf-error-code">1006</span>'
   match = body.match(/<\w+\s+class="cf-error-code">(.*)<\/\w+>/i);
 
   if (match) {
-    var code = parseInt(match[1]);
-    throw new errors.CloudflareError(code, options, response);
+    let code = parseInt(match[1]);
+    throw new CloudflareError(code, options, response);
   }
 
   return false;
 }
 
-function solveChallenge (options, response, body) {
-  var callback = options.callback;
-  var cause;
-  var error;
+function onChallenge (options, response, body) {
+  const callback = options.callback;
+  const uri = response.request.uri;
+  // The query string to send back to Cloudflare
+  const payload = { /* s, jschl_vc, pass, jschl_answer */ };
+
+  let cause;
+  let error;
 
   if (options.challengesToSolve === 0) {
     cause = 'Cloudflare challenge loop';
-    error = new errors.CloudflareError(cause, options, response);
+    error = new CloudflareError(cause, options, response);
     error.errorType = 4;
 
     return callback(error);
   }
 
-  var timeout = parseInt(options.cloudflareTimeout);
-  var uri = response.request.uri;
-  // The query string to send back to Cloudflare
-  // var payload = { s, jschl_vc, pass, jschl_answer };
-  var payload = {};
-  var sandbox;
-  var match;
+  let timeout = parseInt(options.cloudflareTimeout);
+  let match;
 
   match = body.match(/name="s" value="(.+?)"/);
-
   if (match) {
     payload.s = match[1];
   }
 
   match = body.match(/name="jschl_vc" value="(\w+)"/);
-
   if (!match) {
     cause = 'challengeId (jschl_vc) extraction failed';
-    return callback(new errors.ParserError(cause, options, response));
+    return callback(new ParserError(cause, options, response));
   }
 
   payload.jschl_vc = match[1];
 
   match = body.match(/name="pass" value="(.+?)"/);
-
   if (!match) {
     cause = 'Attribute (pass) value extraction failed';
-    return callback(new errors.ParserError(cause, options, response));
+    return callback(new ParserError(cause, options, response));
   }
 
   payload.pass = match[1];
 
   match = body.match(/getElementById\('cf-content'\)[\s\S]+?setTimeout.+?\r?\n([\s\S]+?a\.value\s*=.+?)\r?\n(?:[^{<>]*},\s*(\d{4,}))?/);
-
   if (!match) {
     cause = 'setTimeout callback extraction failed';
-    return callback(new errors.ParserError(cause, options, response));
+    return callback(new ParserError(cause, options, response));
   }
 
   if (isNaN(timeout)) {
-    if (match.length > 2) {
+    if (match[2] !== undefined) {
       timeout = parseInt(match[2]);
 
       if (timeout > options.cloudflareMaxTimeout) {
@@ -297,7 +302,7 @@ function solveChallenge (options, response, body) {
       }
     } else {
       cause = 'Failed to parse challenge timeout';
-      return callback(new errors.ParserError(cause, options, response));
+      return callback(new ParserError(cause, options, response));
     }
   }
 
@@ -305,16 +310,16 @@ function solveChallenge (options, response, body) {
   response.challenge = match[1] + '; a.value';
 
   try {
-    sandbox = createSandbox({ uri: uri, body: body });
-    payload.jschl_answer = vm.runInNewContext(response.challenge, sandbox, VM_OPTIONS);
+    const ctx = new sandbox.Context({ hostname: uri.hostname, body });
+    payload.jschl_answer = sandbox.eval(response.challenge, ctx);
   } catch (error) {
     error.message = 'Challenge evaluation failed: ' + error.message;
-    return callback(new errors.ParserError(error, options, response));
+    return callback(new ParserError(error, options, response));
   }
 
   if (isNaN(payload.jschl_answer)) {
     cause = 'Challenge answer is not a number';
-    return callback(new errors.ParserError(cause, options, response));
+    return callback(new ParserError(cause, options, response));
   }
 
   // Prevent reusing the headers object to simplify unit testing.
@@ -328,44 +333,151 @@ function solveChallenge (options, response, body) {
   }
   // Set the query string and decrement the number of challenges to solve.
   options.qs = payload;
-  options.challengesToSolve = options.challengesToSolve - 1;
+  options.challengesToSolve -= 1;
 
   // Make request with answer after delay.
   timeout -= Date.now() - response.responseStartTime;
   setTimeout(performRequest, timeout, options, false);
 }
 
-function setCookieAndReload (options, response, body) {
-  var callback = options.callback;
+// Parses the reCAPTCHA form and hands control over to the user
+function onCaptcha (options, response, body) {
+  const callback = options.callback;
+  // UDF that has the responsibility of returning control back to cloudscraper
+  const handler = options.onCaptcha;
+  // The form data to send back to Cloudflare
+  const payload = { /* s, g-re-captcha-response */ };
+
+  let cause;
+  let match;
+
+  match = body.match(/<form(?: [^<>]*)? id=["']?challenge-form['"]?(?: [^<>]*)?>([\S\s]*?)<\/form>/);
+  if (!match) {
+    cause = 'Challenge form extraction failed';
+    return callback(new ParserError(cause, options, response));
+  }
+
+  // Defining response.challengeForm for debugging purposes
+  const form = response.challengeForm = match[1];
+
+  match = form.match(/\/recaptcha\/api\/fallback\?k=([^\s"'<>]*)/);
+  if (!match) {
+    // The site key wasn't inside the form so search the entire document
+    match = body.match(/data-sitekey=["']?([^\s"'<>]*)/);
+    if (!match) {
+      cause = 'Unable to find the reCAPTCHA site key';
+      return callback(new ParserError(cause, options, response));
+    }
+  }
+
+  // Everything that is needed to solve the reCAPTCHA
+  response.captcha = { siteKey: match[1], form: payload };
+
+  // Adding formData
+  match = form.match(/<input(?: [^<>]*)? name=[^<>]+>/g);
+  if (!match) {
+    cause = 'Challenge form is missing inputs';
+    return callback(new ParserError(cause, options, response));
+  }
+
+  const inputs = match;
+  // Only adding inputs that have both a name and value defined
+  for (let name, value, i = 0; i < inputs.length; i++) {
+    name = inputs[i].match(/name=["']?([^\s"'<>]*)/);
+    if (name) {
+      value = inputs[i].match(/value=["']?([^\s"'<>]*)/);
+      if (value) {
+        payload[name[1]] = value[1];
+      }
+    }
+  }
+
+  // Sanity check
+  if (!payload['s']) {
+    cause = 'Challenge form is missing secret input';
+    return callback(new ParserError(cause, options, response));
+  }
+
+  // The callback used to green light form submission
+  const submit = function (error) {
+    if (error) {
+      // Pass an user defined error back to the original request call
+      return callback(new CaptchaError(error, options, response));
+    }
+
+    onSubmitCaptcha(options, response, body);
+  };
+
+  // This seems like an okay-ish API (fewer arguments to the handler)
+  response.captcha.submit = submit;
+
+  // We're handing control over to the user now.
+  const thenable = handler(options, response, body);
+  // Handle the case where the user returns a promise
+  if (thenable && typeof thenable.then === 'function') {
+    // eslint-disable-next-line promise/catch-or-return
+    thenable.then(submit, function (error) {
+      if (!error) {
+        // The user broke their promise with a falsy error
+        submit(new Error('Falsy error'));
+      } else {
+        submit(error);
+      }
+    });
+  }
+}
+
+function onSubmitCaptcha (options, response) {
+  const callback = options.callback;
+  const uri = response.request.uri;
+
+  if (!response.captcha.form['g-recaptcha-response']) {
+    const cause = 'Form submission without g-recaptcha-response';
+    return callback(new CaptchaError(cause, options, response));
+  }
+
+  options.method = 'GET';
+  options.qs = response.captcha.form;
+  // Prevent reusing the headers object to simplify unit testing.
+  options.headers = Object.assign({}, options.headers);
+  // Use the original uri as the referer and to construct the form action.
+  options.headers['Referer'] = uri.href;
+  options.uri = uri.protocol + '//' + uri.host + '/cdn-cgi/l/chk_captcha';
+
+  performRequest(options, false);
+}
 
-  var match = body.match(/S='([^']+)'/);
+function onRedirectChallenge (options, response, body) {
+  const callback = options.callback;
+  const uri = response.request.uri;
 
+  const match = body.match(/S='([^']+)'/);
   if (!match) {
-    var cause = 'Cookie code extraction failed';
-    return callback(new errors.ParserError(cause, options, response));
+    const cause = 'Cookie code extraction failed';
+    return callback(new ParserError(cause, options, response));
   }
 
-  var base64EncodedCode = match[1];
+  const base64EncodedCode = match[1];
   response.challenge = Buffer.from(base64EncodedCode, 'base64').toString('ascii');
 
   try {
-    var sandbox = createSandbox({});
     // Evaluate cookie setting code
-    vm.runInNewContext(response.challenge, sandbox, VM_OPTIONS);
+    const ctx = new sandbox.Context();
+    sandbox.eval(response.challenge, ctx);
 
-    options.jar.setCookie(sandbox.document.cookie, response.request.uri.href, { ignoreError: true });
+    options.jar.setCookie(ctx.document.cookie, uri.href, { ignoreError: true });
   } catch (error) {
     error.message = 'Cookie code evaluation failed: ' + error.message;
-    return callback(new errors.ParserError(error, options, response));
+    return callback(new ParserError(error, options, response));
   }
 
-  options.challengesToSolve = options.challengesToSolve - 1;
+  options.challengesToSolve -= 1;
 
   performRequest(options, false);
 }
 
-function processResponseBody (options, response, body) {
-  var callback = options.callback;
+function onRequestComplete (options, response, body) {
+  const callback = options.callback;
 
   if (typeof options.realEncoding === 'string') {
     body = body.toString(options.realEncoding);
@@ -381,47 +493,3 @@ function processResponseBody (options, response, body) {
 
   callback(null, response, body);
 }
-
-function createSandbox (options) {
-  if (options.body) {
-    var body = options.body;
-    var href = 'http://' + options.uri.hostname + '/';
-    var cache = Object.create(null);
-    var keys = [];
-
-    // Sandbox for standard IUAM JS challenge
-    return Object.assign({
-      atob: function (str) {
-        return Buffer.from(str, 'base64').toString('binary');
-      },
-      document: {
-        createElement: function () {
-          return { firstChild: { href: href } };
-        },
-        getElementById: function (id) {
-          if (keys.indexOf(id) === -1) {
-            var re = new RegExp(' id=[\'"]?' + id + '[^>]*>([^<]+)');
-            var match = body.match(re);
-
-            keys.push(id);
-            cache[id] = match === null ? match : { innerHTML: match[1] };
-          }
-
-          return cache[id];
-        }
-      }
-    }, options.context);
-  }
-
-  // Sandbox used in setCookieAndReload
-  return Object.assign({
-    location: {
-      reload: function () {}
-    },
-    document: {}
-  }, options.context);
-}
-
-function randomUA () {
-  return USER_AGENTS[Math.floor(Math.random() * USER_AGENTS.length)];
-}
diff --git a/lib/brotli.js b/lib/brotli.js
new file mode 100644
index 0000000..a1ddcf9
--- /dev/null
+++ b/lib/brotli.js
@@ -0,0 +1,36 @@
+'use strict';
+
+const zlib = require('zlib');
+
+const brotli = module.exports;
+// Convenience boolean used to check for brotli support
+brotli.isAvailable = true;
+// Exported for tests
+brotli.optional = optional;
+
+// Check for node's built-in brotli support
+if (typeof zlib.brotliDecompress === 'function') {
+  brotli.decompress = function (buf) {
+    return zlib.brotliDecompressSync(buf);
+  };
+} else {
+  optional(require);
+}
+
+function optional (require) {
+  try {
+    // Require the NPM installed brotli
+    const decompress = require('brotli/decompress');
+
+    brotli.decompress = function (buf) {
+      return Buffer.from(decompress(buf));
+    };
+  } catch (error) {
+    brotli.isAvailable = false;
+
+    // Don't throw an exception if the module is not installed
+    if (error.code !== 'MODULE_NOT_FOUND') {
+      throw error;
+    }
+  }
+}
diff --git a/lib/browsers.json b/lib/browsers.json
new file mode 100644
index 0000000..c3eab31
--- /dev/null
+++ b/lib/browsers.json
@@ -0,0 +1,336 @@
+{
+  "chrome": [
+    {
+      "User-Agent": [
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.82 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.101 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.101 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.101 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.59 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.75 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.110 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.110 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.110 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.110 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.81 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.113 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.89 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36"
+      ],
+      "headers": {
+        "Connection": "keep-alive",
+        "Upgrade-Insecure-Requests": "1",
+        "User-Agent": null,
+        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
+        "Accept-Language": "en-US,en;q=0.8",
+        "Accept-Encoding": "gzip, deflate, , br"
+      }
+    },
+    {
+      "User-Agent": [
+        "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.86 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.78 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36",
+        "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.79 Safari/537.36"
+      ],
+      "headers": {
+        "Connection": "keep-alive",
+        "Upgrade-Insecure-Requests": "1",
+        "User-Agent": null,
+        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
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+        "Mozilla/5.0 (Linux; Android 5.0; Nexus 6 Build/LRX21O) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 9; Pixel 3 XL Build/PD1A.180720.030) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 9; Pixel 3 Build/PD1A.180720.030) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 9; Pixel 2 Build/PPR1.180610.009) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 4.4; Nexus 5 Build/KRT16M) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 4.4.2; SM-T530 Build/KOT49H) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 4.4.4; SM-N910C Build/KTU84P) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 5.1.1; Nexus 9 Build/LMY47X) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 7.1.1; SM-N950F) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.90 Mobile Safari/537.36"
+      ],
+      "headers": {
+        "Connection": "keep-alive",
+        "Upgrade-Insecure-Requests": "1",
+        "User-Agent": null,
+        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
+        "Accept-Encoding": "gzip, deflate, br",
+        "Accept-Language": "en-US,en;q=0.9"
+      }
+    },
+    {
+      "User-Agent": [
+        "Mozilla/5.0 (Linux; Android 8.1.0; SM-T835 Build/M1AJQ) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 8.0.0; SM-G960F Build/R16NW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36",
+        "Mozilla/5.0 (Linux; Android 5.0; XT1092 Build/LXE22.46-19) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.85 Mobile Safari/537.36"
+      ],
+      "headers": {
+        "Connection": "keep-alive",
+        "Upgrade-Insecure-Requests": "1",
+        "User-Agent": null,
+        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
+        "Accept-Encoding": "gzip, deflate, br",
+        "Accept-Language": "en-GB,en-US;q=0.9,en;q=0.8"
+      }
+    }
+  ]
+}
diff --git a/lib/email-decode.js b/lib/email-decode.js
index 87c2e9c..900acf1 100644
--- a/lib/email-decode.js
+++ b/lib/email-decode.js
@@ -1,4 +1,6 @@
-var pattern = (
+'use strict';
+
+const pattern = (
   // Opening tag
   // $1 = TAG_NAME
   '<([a-z]+)(?: [^>]*)?' + '(?:' +
@@ -14,10 +16,10 @@ var pattern = (
   '(?:[^<]*\\/>|[^<]*?<\\/\\1>)' + ')'
 );
 
-var re = new RegExp(pattern, 'gi');
+const re = new RegExp(pattern, 'gi');
 
 module.exports = function (html) {
-  var match, result;
+  let match, result;
 
   re.lastIndex = 0;
 
@@ -36,9 +38,10 @@ module.exports = function (html) {
 };
 
 function decode (hexStr) {
-  var key = parseInt(hexStr.substr(0, 2), 16);
-  var email = '';
+  const key = parseInt(hexStr.substr(0, 2), 16);
+  let email = '';
 
+  // noinspection ES6ConvertVarToLetConst
   for (var codePoint, i = 2; i < hexStr.length; i += 2) {
     codePoint = parseInt(hexStr.substr(i, 2), 16) ^ key;
     email += String.fromCharCode(codePoint);
diff --git a/lib/headers.js b/lib/headers.js
new file mode 100644
index 0000000..c9be0ae
--- /dev/null
+++ b/lib/headers.js
@@ -0,0 +1,26 @@
+'use strict';
+
+const chromeData = require('./browsers').chrome;
+const useBrotli = require('./brotli').isAvailable;
+
+module.exports = function getHeaders (defaults) {
+  const headers = getChromeHeaders(random(chromeData));
+  return Object.assign({}, defaults, headers);
+};
+
+function random (arr) {
+  return arr[Math.floor(Math.random() * arr.length)];
+}
+
+function getChromeHeaders (options) {
+  const { headers } = options;
+
+  headers['User-Agent'] = random(options['User-Agent']);
+
+  if (!useBrotli && headers['Accept-Encoding']) {
+    headers['Accept-Encoding'] =
+      headers['Accept-Encoding'].replace(/,?\s*\bbr\b\s*/i, '');
+  }
+
+  return headers;
+}
diff --git a/lib/sandbox.js b/lib/sandbox.js
new file mode 100644
index 0000000..96e3594
--- /dev/null
+++ b/lib/sandbox.js
@@ -0,0 +1,50 @@
+'use strict';
+
+const vm = require('vm');
+
+const VM_OPTIONS = {
+  filename: 'iuam-challenge.js',
+  contextOrigin: 'cloudflare:iuam-challenge.js',
+  contextCodeGeneration: { strings: true, wasm: false },
+  timeout: 5000
+};
+
+module.exports = { eval: evaluate, Context };
+
+function evaluate (code, ctx) {
+  return vm.runInNewContext(code, ctx, VM_OPTIONS);
+}
+
+// Global context used to evaluate standard IUAM JS challenge
+function Context (options) {
+  if (!options) options = { body: '', hostname: '' };
+
+  const body = options.body;
+  const href = 'http://' + options.hostname + '/';
+  const cache = Object.create(null);
+  const keys = [];
+
+  this.atob = function (str) {
+    return Buffer.from(str, 'base64').toString('binary');
+  };
+
+  // Used for eval during onRedirectChallenge
+  this.location = { reload: function () {} };
+
+  this.document = {
+    createElement: function () {
+      return { firstChild: { href: href } };
+    },
+    getElementById: function (id) {
+      if (keys.indexOf(id) === -1) {
+        const re = new RegExp(' id=[\'"]?' + id + '[^>]*>([^<]*)');
+        const match = body.match(re);
+
+        keys.push(id);
+        cache[id] = match === null ? match : { innerHTML: match[1] };
+      }
+
+      return cache[id];
+    }
+  };
+}
diff --git a/package.json b/package.json
index ba556a9..aff52a5 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "cloudscraper",
-  "version": "3.9.1",
+  "version": "4.0.0",
   "description": "Bypasses cloudflare's anti-ddos page",
   "main": "index.js",
   "engines": {
@@ -38,7 +38,6 @@
   "license": "MIT",
   "homepage": "https://github.com/codemanki/cloudscraper",
   "dependencies": {
-    "request": "^2.88.0",
     "request-promise": "^4.2.4"
   },
   "devDependencies": {
@@ -57,5 +56,9 @@
     "nyc": "^14.0.0",
     "sinon": "^7.2.4",
     "sinon-chai": "^3.3.0"
+  },
+  "peerDependencies": {
+    "request": "^2.88.0",
+    "brotli": "^1.3.2"
   }
 }
diff --git a/test/fixtures/cf_recaptcha_15_04_2019.html b/test/fixtures/cf_recaptcha_15_04_2019.html
new file mode 100644
index 0000000..0d9e7a2
--- /dev/null
+++ b/test/fixtures/cf_recaptcha_15_04_2019.html
@@ -0,0 +1,162 @@
+<!DOCTYPE html>
+<!--[if lt IE 7]> <html class="no-js ie6 oldie" lang="en-US"> <![endif]-->
+<!--[if IE 7]>    <html class="no-js ie7 oldie" lang="en-US"> <![endif]-->
+<!--[if IE 8]>    <html class="no-js ie8 oldie" lang="en-US"> <![endif]-->
+<!--[if gt IE 8]><!--> <html class="no-js" lang="en-US"> <!--<![endif]-->
+<head>
+<title>Attention Required! | Cloudflare</title>
+<meta name="captcha-bypass" id="captcha-bypass" />
+<meta charset="UTF-8" />
+<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+<meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1" />
+<meta name="robots" content="noindex, nofollow" />
+<meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1" />
+<link rel="stylesheet" id="cf_styles-css" href="/cdn-cgi/styles/cf.errors.css" type="text/css" media="screen,projection" />
+<!--[if lt IE 9]><link rel="stylesheet" id='cf_styles-ie-css' href="/cdn-cgi/styles/cf.errors.ie.css" type="text/css" media="screen,projection" /><![endif]-->
+<style type="text/css">body{margin:0;padding:0}</style>
+
+
+<!--[if gte IE 10]><!--><script type="text/javascript" src="/cdn-cgi/scripts/zepto.min.js"></script><!--<![endif]-->
+<!--[if gte IE 10]><!--><script type="text/javascript" src="/cdn-cgi/scripts/cf.common.js"></script><!--<![endif]-->
+
+
+
+
+</head>
+<body>
+  <div id="cf-wrapper">
+    <div class="cf-alert cf-alert-error cf-cookie-error" id="cookie-alert" data-translate="enable_cookies">Please enable cookies.</div>
+    <div id="cf-error-details" class="cf-error-details-wrapper">
+      <div class="cf-wrapper cf-header cf-error-overview">
+        <h1 data-translate="challenge_headline">One more step</h1>
+        <h2 class="cf-subheadline"><span data-translate="complete_sec_check">Please complete the security check to access</span> example-site.dev</h2>
+      </div><!-- /.header -->
+
+      <div class="cf-section cf-highlight cf-captcha-container">
+        <div class="cf-wrapper">
+          <div class="cf-columns two">
+            <div class="cf-column">
+            <div style="position: absolute; top: -250px; left: -250px;"><a href="https://macinline.com/answeradenoidal.php?day=688">table</a></div>
+              <div class="cf-highlight-inverse cf-form-stacked">
+                <form class="challenge-form" id="challenge-form" action="/cdn-cgi/l/chk_captcha" method="get">
+                  <input type="hidden" name="s" value="6b132d85d185a8255f2451d48fe6a8bee7154ea2-1555377580-1800-AQ1azEkeDOnQP5ByOpwUU/RdbKrmMwHYpkaenRvjPXtB0w8Vbjn/Ceg62tfpp/lT799kjDLEMMuDkEMqQ7iO51kniWCQm00BQvDGl+D0h/WvXDWO96YXOUD3qrqUTuzO7QbUOinc8y8kedvOQkr4c0o="></input>
+  <script type="text/javascript" src="/cdn-cgi/scripts/cf.challenge.js" data-type="normal"  data-ray="0000000000000000" async data-sitekey="6LfBixYUAAAAABhdHynFUIMA_sa4s-XsJvnjtgB0"></script>
+  <div class="g-recaptcha"></div>
+  <noscript id="cf-captcha-bookmark" class="cf-captcha-info">
+    <div><div style="width: 302px">
+      <div>
+        <iframe src="https://www.google.com/recaptcha/api/fallback?k=6LfBixYUAAAAABhdHynFUIMA_sa4s-XsJvnjtgB0" frameborder="0" scrolling="no" style="width: 302px; height:422px; border-style: none;"></iframe>
+      </div>
+      <div style="width: 300px; border-style: none; bottom: 12px; left: 25px; margin: 0px; padding: 0px; right: 25px; background: #f9f9f9; border: 1px solid #c1c1c1; border-radius: 3px;">
+        <textarea id="g-recaptcha-response" name="g-recaptcha-response" class="g-recaptcha-response" style="width: 250px; height: 40px; border: 1px solid #c1c1c1; margin: 10px 25px; padding: 0px; resize: none;"></textarea>
+        <input type="submit" value="Submit"></input>
+      </div>
+    </div></div>
+  </noscript>
+</form>
+
+                <script type="text/javascript">
+  (function(){
+    var a = function() {try{return !!window.addEventListener} catch(e) {return !1} },
+    b = function(b, c) {a() ? document.addEventListener("DOMContentLoaded", b, c) : document.attachEvent("onreadystatechange", b)};
+    b(function(){
+      if (!a()) return;
+
+      window.addEventListener("message", handleMessage, false)
+
+      function handleMessage(event) {
+        if (event.data && event.data.type === 'results') {
+          var f = document.getElementById('challenge-form');
+
+          if (f) {
+            addInput(f, 'bf_challenge_id', '342');
+            addInput(f, 'bf_execution_time', event.data.executionTimeMs);
+            addInput(f, 'bf_result_hash', event.data.resultHash);
+          }
+
+          window.removeEventListener("message", handleMessage, false)
+        }
+      }
+
+      function addInput(parent, name, value) {
+        var input = document.createElement('input');
+        input.type = 'hidden';
+        input.name = name;
+        input.value = value;
+        parent.appendChild(input);
+      }
+
+      function withIframe(iframeContent) {
+        var iframe = document.createElement('iframe');
+        iframe.id = 'bf_test_iframe';
+        iframe.style.visibility = 'hidden';
+        document.body.appendChild(iframe);
+        var doc = (iframe.contentWindow || iframe.contentDocument).document;
+        doc.write(iframeContent);
+        doc.close();
+      }
+
+      withIframe("<!DOCTYPE HTML>\n<meta charset=utf-8>\n<html>\n <head>\n  <title><\/title>\n  <script src=\"https:\/\/ajax.cloudflare.com\/cdn-cgi\/scripts\/697236fc\/cloudflare-static\/bot-filter.js\"><\/__script__>\n  \n <\/head>\n <body>\n    <h1><\/h1>\n  \n <\/body>\n<\/html>\n<script>function r(){var r='<?xml version=\"1.0\" encoding=\"UTF-8\"?><root><child1>value1<\/child1><\/root>',e;return(new DOMParser).parseFromString(r,\"text\/xml\")}test(function(){var r='<root xmlns=\"urn:bar\"><outer xmlns=\"\"><inner>value1<\/inner><\/outer><\/root>',root=(new DOMParser).parseFromString(r,\"text\/xml\").documentElement,e=(new XMLSerializer).serializeToString(root);__c$1(e)});<\/__script__>".replace(/\/__script__/g, '/script'));
+
+    }, false);
+  })();
+  </script>
+
+              </div>
+            </div>
+
+            <div class="cf-column">
+              <div class="cf-screenshot-container">
+
+                <span class="cf-no-screenshot"></span>
+
+              </div>
+            </div>
+          </div><!-- /.columns -->
+        </div>
+      </div><!-- /.captcha-container -->
+
+      <div class="cf-section cf-wrapper">
+        <div class="cf-columns two">
+          <div class="cf-column">
+            <h2 data-translate="why_captcha_headline">Why do I have to complete a CAPTCHA?</h2>
+
+            <p data-translate="why_captcha_detail">Completing the CAPTCHA proves you are a human and gives you temporary access to the web property.</p>
+          </div>
+
+          <div class="cf-column">
+            <h2 data-translate="resolve_captcha_headline">What can I do to prevent this in the future?</h2>
+
+
+            <p data-translate="resolve_captcha_antivirus">If you are on a personal connection, like at home, you can run an anti-virus scan on your device to make sure it is not infected with malware.</p>
+
+            <p data-translate="resolve_captcha_network">If you are at an office or shared network, you can ask the network administrator to run a scan across the network looking for misconfigured or infected devices.</p>
+
+          </div>
+        </div>
+      </div><!-- /.section -->
+
+
+      <div class="cf-error-footer cf-wrapper">
+  <p>
+    <span class="cf-footer-item">Cloudflare Ray ID: <strong>0000000000000000</strong></span>
+    <span class="cf-footer-separator">&bull;</span>
+    <span class="cf-footer-item"><span>Your IP</span>: 000.00.000.00</span>
+    <span class="cf-footer-separator">&bull;</span>
+    <span class="cf-footer-item"><span>Performance &amp; security by</span> <a href="https://www.cloudflare.com/5xx-error-landing?utm_source=error_footer" id="brand_link" target="_blank">Cloudflare</a></span>
+
+  </p>
+</div><!-- /.error-footer -->
+
+
+    </div><!-- /#cf-error-details -->
+  </div><!-- /#cf-wrapper -->
+
+  <script type="text/javascript">
+  window._cf_translation = {};
+
+
+</script>
+
+</body>
+</html>
diff --git a/test/fixtures/js_challenge_10_04_2019.html b/test/fixtures/js_challenge_10_04_2019.html
new file mode 100644
index 0000000..03d780d
--- /dev/null
+++ b/test/fixtures/js_challenge_10_04_2019.html
@@ -0,0 +1,96 @@
+<!DOCTYPE HTML>
+<html lang="en-US">
+<head>
+  <meta charset="UTF-8" />
+  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+  <meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1" />
+  <meta name="robots" content="noindex, nofollow" />
+  <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1" />
+  <title>Just a moment...</title>
+  <style type="text/css">
+    html, body {width: 100%; height: 100%; margin: 0; padding: 0;}
+    body {background-color: #ffffff; font-family: Helvetica, Arial, sans-serif; font-size: 100%;}
+    h1 {font-size: 1.5em; color: #404040; text-align: center;}
+    p {font-size: 1em; color: #404040; text-align: center; margin: 10px 0 0 0;}
+    #spinner {margin: 0 auto 30px auto; display: block;}
+    .attribution {margin-top: 20px;}
+    @-webkit-keyframes bubbles { 33%: { -webkit-transform: translateY(10px); transform: translateY(10px); } 66% { -webkit-transform: translateY(-10px); transform: translateY(-10px); } 100% { -webkit-transform: translateY(0); transform: translateY(0); } }
+    @keyframes bubbles { 33%: { -webkit-transform: translateY(10px); transform: translateY(10px); } 66% { -webkit-transform: translateY(-10px); transform: translateY(-10px); } 100% { -webkit-transform: translateY(0); transform: translateY(0); } }
+    .bubbles { background-color: #404040; width:15px; height: 15px; margin:2px; border-radius:100%; -webkit-animation:bubbles 0.6s 0.07s infinite ease-in-out; animation:bubbles 0.6s 0.07s infinite ease-in-out; -webkit-animation-fill-mode:both; animation-fill-mode:both; display:inline-block; }
+  </style>
+
+    <script type="text/javascript">
+  //<![CDATA[
+  (function(){
+    var a = function() {try{return !!window.addEventListener} catch(e) {return !1} },
+    b = function(b, c) {a() ? document.addEventListener("DOMContentLoaded", b, c) : document.attachEvent("onreadystatechange", b)};
+    b(function(){
+      var a = document.getElementById('cf-content');a.style.display = 'block';
+      setTimeout(function(){
+        var s,t,o,p,b,r,e,a,k,i,n,g,f, klqqNbf={"jRr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};
+        g = String.fromCharCode;
+        o = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
+        e = function(s) {
+          s += "==".slice(2 - (s.length & 3));
+          var bm, r = "", r1, r2, i = 0;
+          for (; i < s.length;) {
+              bm = o.indexOf(s.charAt(i++)) << 18 | o.indexOf(s.charAt(i++)) << 12
+                      | (r1 = o.indexOf(s.charAt(i++))) << 6 | (r2 = o.indexOf(s.charAt(i++)));
+              r += r1 === 64 ? g(bm >> 16 & 255)
+                      : r2 === 64 ? g(bm >> 16 & 255, bm >> 8 & 255)
+                      : g(bm >> 16 & 255, bm >> 8 & 255, bm & 255);
+          }
+          return r;
+        };
+        t = document.createElement('div');
+        t.innerHTML="<a href='/'>x</a>";
+        t = t.firstChild.href;r = t.match(/https?:\/\//)[0];
+        t = t.substr(r.length); t = t.substr(0,t.length-1); k = 'cf-dn-WHRNOV';
+        a = document.getElementById('jschl-answer');
+        f = document.getElementById('challenge-form');
+        ;klqqNbf.jRrD*=+((!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+[])+(!+[]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![])+(+!![]))/+((!+[]+!![]+!![]+!![]+[])+(!+[]+!![]+!![])+(!+[]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![]));klqqNbf.jRrD+=+((!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+[])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![])+(!+[]+!![])+(!+[]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]))/(+(+((!+[]+!![]+!![]+!![]+[])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![])+(!+[]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![])))+(function(p){return eval((true+"")[0]+".ch"+(false+"")[1]+(true+"")[1]+Function("return escape")()(("")["italics"]())[2]+"o"+(undefined+"")[2]+(true+"")[3]+"A"+(true+"")[0]+"("+p+")")}(+((!+[]+!![]+!![]+!![]+[])))));klqqNbf.jRr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klqqNbf.jRrD*=+((!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+[])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![])+(!+[]+!![])+(!+[]+!![]+!![]+!![])+(!+[]+!![]+!![]))/+((+!![]+[])+(!+[]+!![]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![]+!![]+!![]));a.value = (+klqqNbf.jRrD).toFixed(10); '; 121'
+        f.action += location.hash;
+        f.submit();
+      }, 4000);
+    }, false);
+  })();
+  //]]>
+</script>
+
+
+</head>
+<body>
+  <table width="100%" height="100%" cellpadding="20">
+    <tr>
+      <td align="center" valign="middle">
+          <div class="cf-browser-verification cf-im-under-attack">
+  <noscript><h1 data-translate="turn_on_js" style="color:#bd2426;">Please turn JavaScript on and reload the page.</h1></noscript>
+  <div id="cf-content" style="display:none">
+
+    <div>
+      <div class="bubbles"></div>
+      <div class="bubbles"></div>
+      <div class="bubbles"></div>
+    </div>
+
+    <p data-translate="process_is_automatic">This process is automatic. Your browser will redirect to your requested content shortly.</p>
+    <p data-translate="allow_5_secs">Please allow up to 5 seconds&hellip;</p>
+  </div>
+
+  <form id="challenge-form" action="/cdn-cgi/l/chk_jschl" method="get">
+    <input type="hidden" name="s" value="f3b4838af97b6cb02b3c8b1e0f149daf27dbee61-1555369946-1800-AakWW8TP/PRVIBQ2t2QmkJFEmb8TAmeIE7/GS7OUCF+d/7LncO0Zwye3YaCZyfhCfRyQogtebFuSWk2ANVV0pDSXqJ/q5qe0URcQQ2NNaGVMuPVrLh/OrUqD2QUPn0dWGA=="></input>
+    <input type="hidden" name="jschl_vc" value="686d6bea02e6d172aa64f102a684228c"/>
+    <input type="hidden" name="pass" value="1555369950.717-6S1r4kzOYK"/>
+    <input type="hidden" id="jschl-answer" name="jschl_answer"/>
+  </form>
+
+  <div style="display:none;visibility:hidden;" id="cf-dn-WHRNOV">+((!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+[])+(!+[]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(+[])+(!+[]+!![]+!![]+!![])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]))/+((!+[]+!![]+!![]+!![]+!![]+!![]+[])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![])+(+!![])+(!+[]+!![]+!![]+!![]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![]+!![]+!![]+!![]+!![])+(!+[]+!![])+(+[]))</div>
+
+</div>
+
+      </td>
+
+    </tr>
+  </table>
+</body>
+</html>
diff --git a/test/helper.js b/test/helper.js
index 0b93e7d..52e457f 100644
--- a/test/helper.js
+++ b/test/helper.js
@@ -5,6 +5,9 @@ var url     = require('url');
 var path    = require('path');
 var express = require('express');
 
+// Clone the default headers for tests
+var defaultHeaders = Object.assign({}, require('../').defaultParams.headers);
+
 // Cache fixtures so they're only read from fs but once
 var cache = {};
 
@@ -19,13 +22,7 @@ var helper = {
       requester: sinon.match.func,
       jar: request.jar(),
       uri: helper.resolve('/test'),
-      headers: {
-        'Connection': 'keep-alive',
-        'User-Agent': sinon.match.string,
-        'Cache-Control': 'private',
-        'Accept': 'application/xml,application/xhtml+xml,text/html;q=0.9, text/plain;q=0.8,image/png,*/*;q=0.5',
-        'Accept-Language': 'en-US,en;q=0.9'
-      },
+      headers: Object.assign({}, defaultHeaders),
       method: 'GET',
       encoding: null,
       realEncoding: 'utf8',
@@ -33,7 +30,8 @@ var helper = {
       cloudflareTimeout: 1,
       cloudflareMaxTimeout: 30000,
       challengesToSolve: 3,
-      decodeEmails: false
+      decodeEmails: false,
+      gzip: true
     };
   },
   getFixture: function (fileName) {
@@ -92,6 +90,10 @@ express.response.sendChallenge = function (fileName) {
   return this.cloudflare().status(503).sendFixture(fileName);
 };
 
+express.response.sendCaptcha = function (fileName) {
+  return this.cloudflare().status(403).sendFixture(fileName);
+};
+
 express.response.endAbruptly = function () {
   this.connection.write(
     'HTTP/1.1 500\r\n' +
diff --git a/test/test-brotli.js b/test/test-brotli.js
new file mode 100644
index 0000000..3608d44
--- /dev/null
+++ b/test/test-brotli.js
@@ -0,0 +1,60 @@
+/* eslint-disable no-unused-expressions */
+/* eslint-env node, mocha */
+'use strict';
+
+const brotli = require('../lib/brotli');
+const helper = require('./helper');
+const zlib   = require('zlib');
+
+const sinon  = require('sinon');
+const expect = require('chai').expect;
+
+(process.env.BROTLI ? describe : describe.skip)('Brotli (lib)', function () {
+  it('should be available', function () {
+    expect(brotli.isAvailable).to.be.true;
+  });
+
+  it('should have a decompress method', function () {
+    expect(brotli.decompress).to.be.a('function');
+  });
+
+  it('decompress() should accept exactly 1 argument', function () {
+    expect(brotli.decompress.length).to.equal(1);
+  });
+
+  it('decompress() should accept buffer as input', function () {
+    const data = Buffer.from([0x0b, 0x01, 0x80, 0x61, 0x62, 0x63, 0x03]);
+    const result = brotli.decompress(data);
+
+    expect(result).to.be.instanceof(Buffer);
+    expect(result.toString('utf8')).to.equal('abc');
+  });
+
+  (zlib.brotliCompressSync ? it : it.skip)('[internal] decompress() should produce the expected result', function () {
+    const input = helper.getFixture('captcha.html');
+    const data = zlib.brotliCompressSync(Buffer.from(input, 'utf8'));
+    const result = brotli.decompress(data);
+
+    expect(result).to.be.instanceof(Buffer);
+    expect(result.toString('utf8')).to.equal(input);
+  });
+
+  (zlib.brotliCompressSync ? it.skip : it)('[external] decompress() should produce the expected result', function () {
+    const input = helper.getFixture('captcha.html');
+    // Try increasing the timeout if this fails on your system.
+    const data = require('brotli').compress(Buffer.from(input, 'utf8'));
+    const result = brotli.decompress(Buffer.from(data));
+
+    expect(result).to.be.instanceof(Buffer);
+    expect(result.toString('utf8')).to.equal(input);
+  });
+
+  it('optional() should throw an error if the module contains an error', function () {
+    const spy = sinon.spy(function () {
+      // This method should throw if called without arguments
+      brotli.optional();
+    });
+
+    expect(spy).to.throw();
+  });
+});
diff --git a/test/test-captcha.js b/test/test-captcha.js
new file mode 100644
index 0000000..d223231
--- /dev/null
+++ b/test/test-captcha.js
@@ -0,0 +1,183 @@
+/* eslint-disable no-unused-expressions */
+/* eslint-env node, mocha */
+'use strict';
+
+const cloudscraper = require('../index');
+const request      = require('request-promise');
+const errors       = require('../errors');
+const helper       = require('./helper');
+const http         = require('http');
+
+const sinon   = require('sinon');
+const expect  = require('chai').expect;
+
+describe('Cloudscraper', function () {
+  let sandbox;
+  let Request;
+  let uri;
+
+  const requestedPage = helper.getFixture('requested_page.html');
+
+  before(function (done) {
+    helper.listen(function () {
+      uri = helper.resolve('/test');
+
+      // Speed up tests
+      cloudscraper.defaultParams.cloudflareTimeout = 1;
+      done();
+    });
+  });
+
+  after(function () {
+    helper.server.close();
+  });
+
+  beforeEach(function () {
+    // Prepare stubbed Request
+    sandbox = sinon.createSandbox();
+    Request = sandbox.spy(request, 'Request');
+  });
+
+  afterEach(function () {
+    helper.reset();
+    sandbox.restore();
+  });
+
+  it('should handle onCaptcha promise being rejected with a falsy error', function (done) {
+    helper.router.get('/test', function (req, res) {
+      res.sendCaptcha('cf_recaptcha_15_04_2019.html');
+    });
+
+    const options = {
+      uri,
+      onCaptcha: function () {
+        // eslint-disable-next-line prefer-promise-reject-errors
+        return Promise.reject();
+      }
+    };
+
+    const promise = cloudscraper.get(options, function (error) {
+      expect(error).to.be.instanceOf(errors.CaptchaError);
+      expect(error.error).to.be.an('error');
+      expect(error).to.have.property('errorType', 1);
+      expect(error.message).to.include('Falsy error');
+    });
+
+    expect(promise).to.be.rejectedWith(errors.CaptchaError).and.notify(done);
+  });
+
+  for (let stage = 0; stage < 4; stage++) {
+    const desc = {
+      0: 'should resolve reCAPTCHA (version as on 10.04.2019) when user calls captcha.submit()',
+      1: 'should callback with an error if user calls captcha.submit(error)',
+      2: 'should resolve reCAPTCHA (version as on 10.04.2019) when the onCaptcha promise resolves',
+      3: 'should callback with an error if the onCaptcha promise is rejected'
+    };
+
+    // Run this test 4 times
+    it(desc[stage], function (done) {
+      const secret = '6b132d85d185a8255f2451d48fe6a8bee7154ea2-1555377580-1800-AQ1azEkeDOnQP5ByOpwUU/RdbKrmMwHYpkaenRvjPXtB0w8Vbjn/Ceg62tfpp/lT799kjDLEMMuDkEMqQ7iO51kniWCQm00BQvDGl+D0h/WvXDWO96YXOUD3qrqUTuzO7QbUOinc8y8kedvOQkr4c0o=';
+      const siteKey = '6LfBixYUAAAAABhdHynFUIMA_sa4s-XsJvnjtgB0';
+      const expectedError = new Error('anti-captcha failed!');
+
+      helper.router
+        .get('/test', function (req, res) {
+          res.sendCaptcha('cf_recaptcha_15_04_2019.html');
+        })
+        .get('/cdn-cgi/l/chk_captcha', function (req, res) {
+          res.send(requestedPage);
+        });
+
+      const onCaptcha = sinon.spy(function (options, response, body) {
+        expect(options).to.be.an('object');
+        expect(response).to.be.instanceof(http.IncomingMessage);
+        expect(body).to.be.a('string');
+
+        sinon.assert.match(response, {
+          isCloudflare: true,
+          isHTML: true,
+          isCaptcha: true,
+          challengeForm: sinon.match.string,
+          captcha: sinon.match.object
+        });
+
+        sinon.assert.match(response.captcha, {
+          form: { s: secret },
+          siteKey: siteKey,
+          submit: sinon.match.func
+        });
+
+        // Simulate what the user should do here
+        response.captcha.form['g-recaptcha-response'] = 'foobar';
+
+        switch (stage) {
+          case 0:
+            // User green lights form submission
+            response.captcha.submit();
+            break;
+          case 1:
+            // User reports an error when solving the reCAPTCHA
+            response.captcha.submit(expectedError);
+            break;
+          case 2:
+            // User green lights form submission by resolving the returned promise
+            return Promise.resolve();
+          case 3:
+            // User reports an error by rejecting the returned promise
+            return Promise.reject(expectedError);
+        }
+      });
+
+      const firstParams = helper.extendParams({ onCaptcha, uri });
+      const secondParams = helper.extendParams({
+        onCaptcha,
+        method: 'GET',
+        uri: helper.resolve('/cdn-cgi/l/chk_captcha'),
+        headers: {
+          Referer: uri
+        },
+        qs: {
+          s: secret,
+          'g-recaptcha-response': 'foobar'
+        }
+      });
+
+      const options = { onCaptcha, uri };
+
+      const promise = cloudscraper.get(options, function (error, response, body) {
+        switch (stage) {
+          case 0:
+          case 2:
+            expect(error).to.be.null;
+
+            expect(onCaptcha).to.be.calledOnce;
+
+            expect(Request).to.be.calledTwice;
+            expect(Request.firstCall).to.be.calledWithExactly(firstParams);
+            expect(Request.secondCall).to.be.calledWithExactly(secondParams);
+
+            expect(body).to.be.equal(requestedPage);
+            break;
+          case 1:
+          case 3:
+            expect(error).to.be.instanceOf(errors.CaptchaError);
+            expect(error.error).to.be.an('error');
+            expect(error).to.have.property('errorType', 1);
+            expect(error.message).to.include(expectedError.message);
+            break;
+        }
+      });
+
+      switch (stage) {
+        case 0:
+        case 2:
+          expect(promise).to.eventually.equal(requestedPage).and.notify(done);
+          break;
+        case 1:
+        case 3:
+          expect(promise).to.be.rejectedWith(errors.CaptchaError).and.notify(done);
+          break;
+      }
+    });
+  }
+});
diff --git a/test/test-emails.js b/test/test-emails.js
index 860bfe4..c66afe2 100644
--- a/test/test-emails.js
+++ b/test/test-emails.js
@@ -2,11 +2,11 @@
 /* eslint-env node, mocha */
 'use strict';
 
-var decode = require('../lib/email-decode');
-var expect = require('chai').expect;
+const decode = require('../lib/email-decode');
+const expect = require('chai').expect;
 
-var EMAIL = 'cloudscraper@example-site.dev';
-var HEX_STRING = '6506090a10011606170415001725001d040815090048160c11004b010013';
+const EMAIL = 'cloudscraper@example-site.dev';
+const HEX_STRING = '6506090a10011606170415001725001d040815090048160c11004b010013';
 
 function genHTML (body) {
   return '<!DOCTYPE html>\n' +
@@ -22,75 +22,75 @@ function genHTML (body) {
   '</html>\n';
 }
 
-describe('Cloudscraper', function () {
+describe('Email (lib)', function () {
   it('should not modify unprotected html', function () {
-    var raw = genHTML('');
+    const raw = genHTML('');
 
     expect(decode(raw)).to.equal(raw);
   });
 
   it('should remove email protection', function () {
-    var protection = '<a data-cfemail="' + HEX_STRING + '">!@#&*9^%()[]/\\</a>';
+    const protection = '<a data-cfemail="' + HEX_STRING + '">!@#&*9^%()[]/\\</a>';
 
     expect(decode(protection)).to.equal(EMAIL);
   });
 
   it('should replace anchors that have a data-cfemail attribute', function () {
-    var protection = '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="' +
+    const protection = '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="' +
       HEX_STRING + '">[email&#160;protected]</a>';
 
-    var raw = genHTML('<p> The email is ' + EMAIL + '</p>');
-    var enc = genHTML('<p> The email is ' + protection + '</p>');
+    const raw = genHTML('<p> The email is ' + EMAIL + '</p>');
+    const enc = genHTML('<p> The email is ' + protection + '</p>');
 
     expect(decode(enc)).to.equal(raw);
   });
 
   it('should replace spans that have a data-cfemail attribute', function () {
-    var protection = '<span href="/cdn-cgi/l/email-protection" data-cfemail="' +
+    const protection = '<span href="/cdn-cgi/l/email-protection" data-cfemail="' +
       HEX_STRING + '">[email&#160;protected]</span>';
 
-    var raw = genHTML('<p> The email is ' + EMAIL + '</p>');
-    var enc = genHTML('<p> The email is ' + protection + '</p>');
+    const raw = genHTML('<p> The email is ' + EMAIL + '</p>');
+    const enc = genHTML('<p> The email is ' + protection + '</p>');
 
     expect(decode(enc)).to.equal(raw);
   });
 
   it('should be space agnostic', function () {
-    var protection = '<NODE    href="/cdn-cgi/l/email-protection"    data-cfemail="' +
+    const protection = '<NODE    href="/cdn-cgi/l/email-protection"    data-cfemail="' +
       HEX_STRING + '"  \r\n>\n[email&#160;protected]\r\n</NODE>';
 
-    var raw = genHTML('\r\n<p>\n The email <br/>is ' + EMAIL + '\r\n</p>\n');
-    var enc = genHTML('\r\n<p>\n The email <br/>is ' + protection + '\r\n</p>\n');
+    const raw = genHTML('\r\n<p>\n The email <br/>is ' + EMAIL + '\r\n</p>\n');
+    const enc = genHTML('\r\n<p>\n The email <br/>is ' + protection + '\r\n</p>\n');
 
     expect(decode(enc)).to.equal(raw);
   });
 
   it('should not replace nodes if they have children', function () {
-    var protection = '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="' +
+    const protection = '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="' +
       HEX_STRING + '"><span>[email&#160;protected]</span></a>';
-    var enc = genHTML('<p> The email is ' + protection + '</p>');
+    const enc = genHTML('<p> The email is ' + protection + '</p>');
 
     expect(decode(enc)).to.equal(enc);
   });
 
   it('should not replace malformed html', function () {
-    var protection = '<a data-cfemail="' + HEX_STRING + '">\n<\n</a>';
-    var enc = genHTML('<p> The email is ' + protection + '</p>');
+    const protection = '<a data-cfemail="' + HEX_STRING + '">\n<\n</a>';
+    const enc = genHTML('<p> The email is ' + protection + '</p>');
 
     expect(decode(enc)).to.equal(enc);
   });
 
   it('should account for self-closing nodes', function () {
-    var protection = '<a data-cfemail="' + HEX_STRING + '"/><span>test</span>';
+    const protection = '<a data-cfemail="' + HEX_STRING + '"/><span>test</span>';
 
     expect(decode(protection)).to.equal(EMAIL + '<span>test</span>');
   });
 
   it('should update href attribute values', function () {
-    var protection = '<a b="c" href="/cdn-cgi/l/email-protection#' + HEX_STRING + '"></a>';
+    const protection = '<a b="c" href="/cdn-cgi/l/email-protection#' + HEX_STRING + '"></a>';
 
-    var raw = genHTML('<a b="c" href="mailto:' + EMAIL + '"></a>');
-    var enc = genHTML(protection);
+    const raw = genHTML('<a b="c" href="mailto:' + EMAIL + '"></a>');
+    const enc = genHTML(protection);
 
     expect(decode(enc)).to.equal(raw);
   });
diff --git a/test/test-errors.js b/test/test-errors.js
index 2b328da..dad5b7b 100644
--- a/test/test-errors.js
+++ b/test/test-errors.js
@@ -2,19 +2,19 @@
 /* eslint-env node, mocha */
 'use strict';
 
-var cloudscraper = require('../index');
-var request      = require('request-promise');
-var helper       = require('./helper');
-var errors       = require('../errors');
+const cloudscraper = require('../index');
+const request      = require('request-promise');
+const helper       = require('./helper');
+const errors       = require('../errors');
 
-var sinon  = require('sinon');
-var expect = require('chai').expect;
-var assert = require('chai').assert;
+const sinon  = require('sinon');
+const expect = require('chai').expect;
+const assert = require('chai').assert;
 
 describe('Cloudscraper', function () {
-  var sandbox;
-  var Request;
-  var uri;
+  let sandbox;
+  let Request;
+  let uri;
 
   before(function (done) {
     helper.listen(function () {
@@ -46,7 +46,7 @@ describe('Cloudscraper', function () {
       res.endAbruptly();
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.RequestError);
       expect(error.error).to.be.an('error');
       expect(error).to.have.property('errorType', 0);
@@ -62,7 +62,7 @@ describe('Cloudscraper', function () {
       res.cloudflare().status(504).end();
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 1, means captcha is served
       expect(error).to.be.instanceOf(errors.CloudflareError);
       expect(error).to.have.property('error', 504);
@@ -82,7 +82,7 @@ describe('Cloudscraper', function () {
       res.sendChallenge('captcha.html');
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 1, means captcha is served
       expect(error).to.be.instanceOf(errors.CaptchaError);
       expect(error).to.have.property('error', 'captcha');
@@ -103,7 +103,7 @@ describe('Cloudscraper', function () {
       res.cloudflare().status(500).sendFixture('access_denied.html');
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 2, means inner cloudflare error
       expect(error).to.be.instanceOf(errors.CloudflareError);
       expect(error).to.have.property('error', 1006);
@@ -117,13 +117,13 @@ describe('Cloudscraper', function () {
   });
 
   it('should add a description to 5xx range cloudflare errors', function (done) {
-    var html = helper.getFixture('access_denied.html').toString('utf8');
+    const html = helper.getFixture('access_denied.html').toString('utf8');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(504).send(html.replace('1006', '504'));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 2, means inner cloudflare error
       expect(error).to.be.instanceOf(errors.CloudflareError);
       expect(error).to.have.property('error', 504);
@@ -137,13 +137,13 @@ describe('Cloudscraper', function () {
   });
 
   it('should not error if error description is unavailable', function (done) {
-    var html = helper.getFixture('access_denied.html').toString('utf8');
+    const html = helper.getFixture('access_denied.html').toString('utf8');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(500).send(html.replace('1006', '5111'));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 2, means inner cloudflare error
       expect(error).to.be.instanceOf(errors.CloudflareError);
       expect(error).to.have.property('error', 5111);
@@ -162,7 +162,7 @@ describe('Cloudscraper', function () {
     });
 
     // The expected params for all subsequent calls to Request
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl')
     });
 
@@ -172,7 +172,7 @@ describe('Cloudscraper', function () {
       qs: sinon.match.object
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.CloudflareError);
       expect(error).to.have.property('error', 'Cloudflare challenge loop');
       expect(error).to.have.property('errorType', 4);
@@ -180,7 +180,8 @@ describe('Cloudscraper', function () {
       assert.equal(Request.callCount, 4, 'Request call count');
       expect(Request.firstCall).to.be.calledWithExactly(helper.defaultParams);
 
-      var total = helper.defaultParams.challengesToSolve + 1;
+      const total = helper.defaultParams.challengesToSolve + 1;
+      // noinspection ES6ConvertVarToLetConst
       for (var i = 1; i < total; i++) {
         // Decrement the number of challengesToSolve to match actual params
         expectedParams.challengesToSolve -= 1;
@@ -196,10 +197,10 @@ describe('Cloudscraper', function () {
       res.status(503).end();
     });
 
-    var expectedParams = helper.extendParams({ json: true });
-    var options = { uri: uri, json: true };
+    const expectedParams = helper.extendParams({ json: true });
+    const options = { uri: uri, json: true };
 
-    var promise = cloudscraper.get(options, function (error) {
+    const promise = cloudscraper.get(options, function (error) {
       expect(error).to.be.instanceOf(errors.RequestError);
       expect(error).to.have.property('error', null);
       expect(error).to.have.property('errorType', 0);
@@ -218,7 +219,7 @@ describe('Cloudscraper', function () {
       res.sendChallenge('invalid_js_challenge.html');
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error').that.is.ok;
       expect(error).to.have.property('errorType', 3);
@@ -230,14 +231,14 @@ describe('Cloudscraper', function () {
   });
 
   it('should return error if js challenge has error during evaluation', function (done) {
-    var html = helper.getFixture('js_challenge_03_12_2018_1.html');
+    const html = helper.getFixture('js_challenge_03_12_2018_1.html');
 
     helper.router.get('/test', function (req, res) {
       // Adds a syntax error near the end of line 37
       res.cloudflare().status(503).send(html.replace(/\.toFixed/gm, '..toFixed'));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error').that.is.an('error');
       expect(error).to.have.property('errorType', 3);
@@ -250,13 +251,13 @@ describe('Cloudscraper', function () {
   });
 
   it('should return error if pass extraction fails', function (done) {
-    var html = helper.getFixture('js_challenge_03_12_2018_1.html');
+    const html = helper.getFixture('js_challenge_03_12_2018_1.html');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(503).send(html.replace(/name="pass"/gm, ''));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error', 'Attribute (pass) value extraction failed');
       expect(error).to.have.property('errorType', 3);
@@ -268,13 +269,13 @@ describe('Cloudscraper', function () {
   });
 
   it('should return error if challengeId extraction fails', function (done) {
-    var html = helper.getFixture('js_challenge_03_12_2018_1.html');
+    const html = helper.getFixture('js_challenge_03_12_2018_1.html');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(503).send(html.replace(/name="jschl_vc"/gm, ''));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error', 'challengeId (jschl_vc) extraction failed');
       expect(error).to.have.property('errorType', 3);
@@ -286,14 +287,14 @@ describe('Cloudscraper', function () {
   });
 
   it('should return error if challenge answer is not a number', function (done) {
-    var html = helper.getFixture('js_challenge_03_12_2018_1.html');
+    const html = helper.getFixture('js_challenge_03_12_2018_1.html');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(503)
         .send(html.replace(/a.value.*/, 'a.value="abc" + t.length'));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error', 'Challenge answer is not a number');
       expect(error).to.have.property('errorType', 3);
@@ -313,7 +314,7 @@ describe('Cloudscraper', function () {
         res.endAbruptly();
       });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 0, a connection error for example
       expect(error).to.be.instanceOf(errors.RequestError);
       expect(error.error).to.be.an('error');
@@ -336,7 +337,7 @@ describe('Cloudscraper', function () {
       });
 
     // Second call to request.get returns recaptcha
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       challengesToSolve: 2
     });
@@ -347,7 +348,7 @@ describe('Cloudscraper', function () {
       qs: sinon.match.object
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       // errorType 1, means captcha is served
       expect(error).to.be.instanceOf(errors.CaptchaError);
       expect(error).to.have.property('error', 'captcha');
@@ -362,14 +363,14 @@ describe('Cloudscraper', function () {
   });
 
   it('should return error if challenge page cookie extraction fails', function (done) {
-    var html = helper.getFixture('js_challenge_cookie.html').toString('utf8');
+    const html = helper.getFixture('js_challenge_cookie.html').toString('utf8');
 
     helper.router.get('/test', function (req, res) {
       // The cookie extraction codes looks for the `S` variable assignment
       res.cloudflare().status(503).send(html.replace(/S=/gm, 'Z='));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error', 'Cookie code extraction failed');
       expect(error).to.have.property('errorType', 3);
@@ -381,9 +382,9 @@ describe('Cloudscraper', function () {
   });
 
   it('should throw a TypeError if callback is not a function', function (done) {
-    var spy = sinon.spy(function () {
+    const spy = sinon.spy(function () {
       // request-promise always provides a callback so change requester
-      var options = { uri: uri, requester: require('request') };
+      const options = { uri: uri, requester: require('request') };
       cloudscraper.get(options);
     });
 
@@ -392,7 +393,7 @@ describe('Cloudscraper', function () {
   });
 
   it('should throw a TypeError if requester is not a function', function (done) {
-    var spy = sinon.spy(function () {
+    const spy = sinon.spy(function () {
       cloudscraper.get({ requester: null });
     });
 
@@ -401,8 +402,8 @@ describe('Cloudscraper', function () {
   });
 
   it('should throw a TypeError if challengesToSolve is not a number', function (done) {
-    var spy = sinon.spy(function () {
-      var options = { uri: uri, challengesToSolve: 'abc' };
+    const spy = sinon.spy(function () {
+      const options = { uri: uri, challengesToSolve: 'abc' };
 
       cloudscraper.get(options);
     });
@@ -412,8 +413,8 @@ describe('Cloudscraper', function () {
   });
 
   it('should throw a TypeError if cloudflareMaxTimeout is not a number', function (done) {
-    var spy = sinon.spy(function () {
-      var options = { uri: uri, cloudflareMaxTimeout: 'abc' };
+    const spy = sinon.spy(function () {
+      const options = { uri: uri, cloudflareMaxTimeout: 'abc' };
 
       cloudscraper.get(options, function () {});
     });
@@ -424,14 +425,14 @@ describe('Cloudscraper', function () {
 
   it('should return error if cookie setting code evaluation fails', function (done) {
     // Change the cookie setting code so the vm will throw an error
-    var html = helper.getFixture('js_challenge_cookie.html').toString('utf8');
-    var b64 = Buffer.from('throw new Error(\'vm eval failed\');').toString('base64');
+    const html = helper.getFixture('js_challenge_cookie.html').toString('utf8');
+    const b64 = Buffer.from('throw new Error(\'vm eval failed\');').toString('base64');
 
     helper.router.get('/test', function (req, res) {
       res.cloudflare().status(503).send(html.replace(/S='([^']+)'/, 'S=\'' + b64 + '\''));
     });
 
-    var promise = cloudscraper.get(uri, function (error) {
+    const promise = cloudscraper.get(uri, function (error) {
       expect(error).to.be.instanceOf(errors.ParserError);
       expect(error).to.have.property('error').that.is.an('error');
       expect(error).to.have.property('errorType', 3);
@@ -444,14 +445,14 @@ describe('Cloudscraper', function () {
   });
 
   it('should not error if Error.captureStackTrace is undefined', function () {
-    var desc = Object.getOwnPropertyDescriptor(Error, 'captureStackTrace');
+    const desc = Object.getOwnPropertyDescriptor(Error, 'captureStackTrace');
 
     Object.defineProperty(Error, 'captureStackTrace', {
       configurable: true,
       value: undefined
     });
 
-    var spy = sinon.spy(function () {
+    const spy = sinon.spy(function () {
       throw new errors.RequestError();
     });
 
diff --git a/test/test-headers.js b/test/test-headers.js
new file mode 100644
index 0000000..dc7beaa
--- /dev/null
+++ b/test/test-headers.js
@@ -0,0 +1,47 @@
+/* eslint-disable no-unused-expressions */
+/* eslint-env node, mocha */
+'use strict';
+
+const getHeaders = require('../lib/headers');
+
+const sinon  = require('sinon');
+const expect = require('chai').expect;
+
+describe('Headers (lib)', function () {
+  const browsers = require('../lib/browsers');
+
+  it('default export should be a function', function () {
+    expect(getHeaders).to.be.a('function');
+  });
+
+  it('should always return an object with user agent', function () {
+    for (let i = 0; i < 100; i++) {
+      sinon.assert.match(getHeaders(), { 'User-Agent': sinon.match.string });
+    }
+
+    browsers.chrome.forEach(function (options) {
+      try {
+        expect(options['User-Agent']).to.be.an('array');
+        expect(options['User-Agent'].length).to.be.above(0);
+      } catch (error) {
+        error.message += '\n\n' + JSON.stringify(options, null, 2);
+        throw error;
+      }
+    });
+  });
+
+  it('should always retain insertion order', function () {
+    for (let keys, i = 0; i < 100; i++) {
+      keys = Object.keys(getHeaders({ Host: 'foobar' }));
+      expect(keys[0]).to.equal('Host');
+      expect(keys[1]).to.equal('Connection');
+    }
+
+    for (let keys, i = 0; i < 100; i++) {
+      keys = Object.keys(getHeaders({ Host: 'foobar', 'N/A': null }));
+      expect(keys[0]).to.equal('Host');
+      expect(keys[1]).to.equal('N/A');
+      expect(keys[2]).to.equal('Connection');
+    }
+  });
+});
diff --git a/test/test-index.js b/test/test-index.js
index 7c0178f..26bb747 100644
--- a/test/test-index.js
+++ b/test/test-index.js
@@ -2,20 +2,20 @@
 /* eslint-env node, mocha */
 'use strict';
 
-var cloudscraper = require('../index');
-var request      = require('request-promise');
-var helper       = require('./helper');
-var querystring  = require('querystring');
+const cloudscraper = require('../index');
+const request      = require('request-promise');
+const helper       = require('./helper');
+const querystring  = require('querystring');
 
-var sinon   = require('sinon');
-var expect  = require('chai').expect;
+const sinon   = require('sinon');
+const expect  = require('chai').expect;
 
 describe('Cloudscraper', function () {
-  var sandbox;
-  var Request;
-  var uri;
+  let sandbox;
+  let Request;
+  let uri;
 
-  var requestedPage = helper.getFixture('requested_page.html');
+  const requestedPage = helper.getFixture('requested_page.html');
 
   before(function (done) {
     helper.listen(function () {
@@ -43,16 +43,16 @@ describe('Cloudscraper', function () {
   });
 
   it('should return requested page, in the specified encoding', function (done) {
-    var expectedBody = Buffer.from(requestedPage).toString('utf16le');
+    const expectedBody = Buffer.from(requestedPage).toString('utf16le');
 
     helper.router.get('/test', function (req, res) {
       res.send(requestedPage);
     });
 
-    var expectedParams = helper.extendParams({ realEncoding: 'utf16le' });
-    var options = { uri: uri, encoding: 'utf16le' };
+    const expectedParams = helper.extendParams({ realEncoding: 'utf16le' });
+    const options = { uri: uri, encoding: 'utf16le' };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.equal(expectedBody);
@@ -62,16 +62,16 @@ describe('Cloudscraper', function () {
   });
 
   it('should return json', function (done) {
-    var expectedBody = { a: 'test' };
+    const expectedBody = { a: 'test' };
 
     helper.router.get('/test', function (req, res) {
       res.send(expectedBody);
     });
 
-    var expectedParams = helper.extendParams({ json: true });
-    var options = { uri: uri, json: true };
+    const expectedParams = helper.extendParams({ json: true });
+    const options = { uri: uri, json: true };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.eql(expectedBody);
@@ -86,10 +86,10 @@ describe('Cloudscraper', function () {
     });
 
     // Disable status code checking
-    var expectedParams = helper.extendParams({ simple: false });
-    var options = { uri: uri, simple: false };
+    const expectedParams = helper.extendParams({ simple: false });
+    const options = { uri: uri, simple: false };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.equal('xyz');
@@ -103,7 +103,7 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(helper.defaultParams);
       expect(body).to.be.equal(requestedPage);
@@ -113,13 +113,13 @@ describe('Cloudscraper', function () {
   });
 
   it('should not trigger any error if recaptcha is present in page not protected by CF', function (done) {
-    var expectedBody = helper.getFixture('page_with_recaptcha.html');
+    const expectedBody = helper.getFixture('page_with_recaptcha.html');
 
     helper.router.get('/test', function (req, res) {
       res.send(expectedBody);
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(helper.defaultParams);
       expect(body).to.be.equal(expectedBody);
@@ -138,7 +138,7 @@ describe('Cloudscraper', function () {
       });
 
     // Second call to Request will have challenge solution
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
         'jschl_vc': '89cdff5eaa25923e0f26e29e5195dce9',
@@ -152,7 +152,7 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -176,7 +176,7 @@ describe('Cloudscraper', function () {
       });
 
     // Second call to Request will have challenge solution
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
         'jschl_vc': '346b959db0cfa38f9938acc11d6e1e6e',
@@ -190,7 +190,7 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -214,7 +214,7 @@ describe('Cloudscraper', function () {
       });
 
     // Second call to Request will have challenge solution
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
         'jschl_vc': '18e0eb4e7cc844880cd9822df9d8546e',
@@ -228,7 +228,7 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -251,7 +251,7 @@ describe('Cloudscraper', function () {
       });
 
     // Second call to Request will have challenge solution
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
         's': '08ee9f79382c9f784ef868f239a0984261a28b2f-1553213547-1800-AXjMT2d0Sx0fifn2gHCBp7sjO3hmbH5Pab9lPE92HxBLetotfG2HQ0U8ioQ2CJwOMGV5pmmBmffUDmmyxIyCuRCBOxecZXzYCBZZReVFCTXgIlpXL8ZcztRhE9Bm3BNGfg==',
@@ -265,7 +265,43 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
+      expect(error).to.be.null;
+
+      expect(Request).to.be.calledTwice;
+      expect(Request.firstCall).to.be.calledWithExactly(helper.defaultParams);
+      expect(Request.secondCall).to.be.calledWithExactly(expectedParams);
+
+      expect(body).to.be.equal(requestedPage);
+    });
+
+    expect(promise).to.eventually.equal(requestedPage).and.notify(done);
+  });
+
+  it('should resolve challenge (version as on 10.04.2019) and then return page', function (done) {
+    helper.router
+      .get('/test', function (req, res) {
+        res.sendChallenge('js_challenge_10_04_2019.html');
+      })
+      .get('/cdn-cgi/l/chk_jschl', function (req, res) {
+        res.send(requestedPage);
+      });
+
+    const expectedParams = helper.extendParams({
+      uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
+      qs: {
+        's': 'f3b4838af97b6cb02b3c8b1e0f149daf27dbee61-1555369946-1800-AakWW8TP/PRVIBQ2t2QmkJFEmb8TAmeIE7/GS7OUCF+d/7LncO0Zwye3YaCZyfhCfRyQogtebFuSWk2ANVV0pDSXqJ/q5qe0URcQQ2NNaGVMuPVrLh/OrUqD2QUPn0dWGA==',
+        'jschl_vc': '686d6bea02e6d172aa64f102a684228c',
+        'jschl_answer': String(9.8766929385 + helper.uri.hostname.length),
+        'pass': '1555369950.717-6S1r4kzOYK'
+      },
+      headers: {
+        'Referer': uri
+      },
+      challengesToSolve: 2
+    });
+
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -280,7 +316,7 @@ describe('Cloudscraper', function () {
 
   it('should resolve 2 consequent challenges', function (done) {
     // Cloudflare is enabled for site. It returns a page with JS challenge
-    var additionalChallenge = true;
+    let additionalChallenge = true;
 
     helper.router
       .get('/test', function (req, res) {
@@ -296,8 +332,8 @@ describe('Cloudscraper', function () {
         }
       });
 
-    var firstParams  = helper.extendParams({ resolveWithFullResponse: true });
-    var secondParams = helper.extendParams({
+    const firstParams  = helper.extendParams({ resolveWithFullResponse: true });
+    const secondParams = helper.extendParams({
       resolveWithFullResponse: true,
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
@@ -312,7 +348,7 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var thirdParams = helper.extendParams({
+    const thirdParams = helper.extendParams({
       resolveWithFullResponse: true,
       uri: helper.resolve('/cdn-cgi/l/chk_jschl'),
       qs: {
@@ -328,9 +364,9 @@ describe('Cloudscraper', function () {
       challengesToSolve: 1
     });
 
-    var options = { uri: uri, resolveWithFullResponse: true };
+    const options = { uri: uri, resolveWithFullResponse: true };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledThrice;
@@ -349,16 +385,16 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var formData = { some: 'data' };
+    const formData = { some: 'data' };
 
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       method: 'POST',
       formData: formData
     });
 
-    var options = { uri: uri, formData: formData };
+    const options = { uri: uri, formData: formData };
 
-    var promise = cloudscraper.post(options, function (error, response, body) {
+    const promise = cloudscraper.post(options, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.equal(requestedPage);
@@ -372,9 +408,9 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var expectedParams = helper.extendParams({ method: 'DELETE' });
+    const expectedParams = helper.extendParams({ method: 'DELETE' });
 
-    var promise = cloudscraper.delete(uri, function (error, response, body) {
+    const promise = cloudscraper.delete(uri, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.equal(requestedPage);
@@ -388,12 +424,12 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var expectedBody = Buffer.from(requestedPage, 'utf8');
-    var expectedParams = helper.extendParams({ realEncoding: null });
+    const expectedBody = Buffer.from(requestedPage, 'utf8');
+    const expectedParams = helper.extendParams({ realEncoding: null });
 
-    var options = { uri: uri, encoding: null };
+    const options = { uri: uri, encoding: null };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
       expect(body).to.be.eql(expectedBody);
@@ -412,12 +448,12 @@ describe('Cloudscraper', function () {
       }
     });
 
-    var expectedParams = helper.extendParams({ challengesToSolve: 2 });
+    const expectedParams = helper.extendParams({ challengesToSolve: 2 });
 
     // We need to override cloudscraper's default jar for this test
-    var options = { uri: uri, jar: helper.defaultParams.jar };
+    const options = { uri: uri, jar: helper.defaultParams.jar };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -443,12 +479,12 @@ describe('Cloudscraper', function () {
         }
       });
 
-    var firstParams  = helper.extendParams({
+    const firstParams  = helper.extendParams({
       proxy: helper.uri.href,
       uri: 'http://example-site.dev/test'
     });
 
-    var secondParams = helper.extendParams({
+    const secondParams = helper.extendParams({
       proxy: helper.uri.href,
       uri: 'http://example-site.dev/cdn-cgi/l/chk_jschl',
       qs: {
@@ -463,12 +499,12 @@ describe('Cloudscraper', function () {
       challengesToSolve: 2
     });
 
-    var options = {
+    const options = {
       proxy: helper.uri.href,
       uri: 'http://example-site.dev/test'
     };
 
-    var promise = cloudscraper.get(options, function (error, response, body) {
+    const promise = cloudscraper.get(options, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -491,16 +527,16 @@ describe('Cloudscraper', function () {
       }
     });
 
-    var customJar = request.jar();
+    const customJar = request.jar();
 
-    var firstParams  = helper.extendParams({ jar: customJar });
-    var secondParams = helper.extendParams({
+    const firstParams  = helper.extendParams({ jar: customJar });
+    const secondParams = helper.extendParams({
       jar: customJar,
       challengesToSolve: 2
     });
 
     // We need to override cloudscraper's default jar for this test
-    var options = { uri: uri, jar: customJar };
+    const options = { uri: uri, jar: customJar };
 
     customJar.setCookie('custom cookie', 'http://custom-site.dev/');
 
@@ -513,7 +549,7 @@ describe('Cloudscraper', function () {
 
       expect(body).to.be.equal(requestedPage);
 
-      var customCookie = customJar.getCookieString('http://custom-site.dev/');
+      let customCookie = customJar.getCookieString('http://custom-site.dev/');
       expect(customCookie).to.equal('custom cookie');
 
       cloudscraper.get(options, function (error) {
@@ -531,7 +567,7 @@ describe('Cloudscraper', function () {
   it('should define custom defaults function', function (done) {
     expect(cloudscraper.defaults).to.not.equal(request.defaults);
 
-    var custom = cloudscraper.defaults({ challengesToSolve: 5 });
+    const custom = cloudscraper.defaults({ challengesToSolve: 5 });
     expect(custom.defaults).to.equal(cloudscraper.defaults);
     done();
   });
@@ -545,11 +581,11 @@ describe('Cloudscraper', function () {
         res.sendFixture('page_with_emails.html');
       });
 
-    var cf = cloudscraper.defaults({ decodeEmails: true });
+    const cf = cloudscraper.defaults({ decodeEmails: true });
 
-    var firstParams = helper.extendParams({ decodeEmails: true });
+    const firstParams = helper.extendParams({ decodeEmails: true });
 
-    var promise = cf.get(uri, function (error, response, body) {
+    const promise = cf.get(uri, function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -570,14 +606,14 @@ describe('Cloudscraper', function () {
         res.send(requestedPage);
       });
 
-    var cf = cloudscraper.defaults({ baseUrl: helper.uri.href });
+    const cf = cloudscraper.defaults({ baseUrl: helper.uri.href });
 
-    var firstParams = helper.extendParams({
+    const firstParams = helper.extendParams({
       baseUrl: helper.uri.href,
       uri: '/test'
     });
 
-    var promise = cf.get('/test', function (error, response, body) {
+    const promise = cf.get('/test', function (error, response, body) {
       expect(error).to.be.null;
 
       expect(Request).to.be.calledTwice;
@@ -599,16 +635,16 @@ describe('Cloudscraper', function () {
         res.send(requestedPage);
       });
 
-    var expectedParams = helper.extendParams({ cloudflareTimeout: 50 });
+    const expectedParams = helper.extendParams({ cloudflareTimeout: 50 });
 
-    var start = Date.now();
-    var options = { uri: uri, cloudflareTimeout: 50 };
+    const start = Date.now();
+    const options = { uri: uri, cloudflareTimeout: 50 };
 
-    var promise = cloudscraper.get(options, function (error) {
+    const promise = cloudscraper.get(options, function (error) {
       expect(error).to.be.null;
       expect(Request.firstCall).to.be.calledWithExactly(expectedParams);
 
-      var elapsed = Date.now() - start;
+      const elapsed = Date.now() - start;
       // Aiming to be within ~150ms of specified timeout
       expect(elapsed >= 50 && elapsed <= 200).to.be.ok;
     });
@@ -617,8 +653,8 @@ describe('Cloudscraper', function () {
   });
 
   it('sandbox.document.getElementById should not error', function (done) {
-    var html = helper.getFixture('js_challenge_21_03_2019.html');
-    var statements = 'document.getElementById("missing");'.repeat(2);
+    const html = helper.getFixture('js_challenge_21_03_2019.html');
+    const statements = 'document.getElementById("missing");'.repeat(2);
 
     helper.router
       .get('/test', function (req, res) {
@@ -629,7 +665,7 @@ describe('Cloudscraper', function () {
         res.send(requestedPage);
       });
 
-    var promise = cloudscraper.get(uri, function (error, response, body) {
+    const promise = cloudscraper.get(uri, function (error, response, body) {
       expect(error).to.be.null;
       expect(Request).to.be.calledTwice;
     });
diff --git a/test/test-rp.js b/test/test-rp.js
index f53dcdf..9c6b37b 100644
--- a/test/test-rp.js
+++ b/test/test-rp.js
@@ -2,19 +2,19 @@
 /* eslint-env node, mocha */
 'use strict';
 
-var cloudscraper = require('../index');
-var request      = require('request-promise');
-var helper       = require('./helper');
+const cloudscraper = require('../index');
+const request      = require('request-promise');
+const helper       = require('./helper');
 
-var sinon  = require('sinon');
-var expect = require('chai').expect;
+const sinon  = require('sinon');
+const expect = require('chai').expect;
 
 describe('Cloudscraper', function () {
-  var sandbox;
-  var Request;
-  var uri;
+  let sandbox;
+  let Request;
+  let uri;
 
-  var requestedPage = helper.getFixture('requested_page.html');
+  const requestedPage = helper.getFixture('requested_page.html');
 
   before(function (done) {
     helper.listen(function () {
@@ -46,7 +46,7 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var expectedParams = helper.extendParams({ callback: undefined });
+    const expectedParams = helper.extendParams({ callback: undefined });
 
     return cloudscraper.get(uri).then(function (body) {
       expect(Request).to.be.calledOnceWithExactly(expectedParams);
@@ -59,7 +59,7 @@ describe('Cloudscraper', function () {
       res.send(requestedPage);
     });
 
-    var expectedParams = helper.extendParams({
+    const expectedParams = helper.extendParams({
       callback: undefined,
       resolveWithFullResponse: true
     });
@@ -67,7 +67,7 @@ describe('Cloudscraper', function () {
     // The method is implicitly GET
     delete expectedParams.method;
 
-    var options = {
+    const options = {
       uri: uri,
       resolveWithFullResponse: true
     };
@@ -86,7 +86,7 @@ describe('Cloudscraper', function () {
       res.endAbruptly();
     });
 
-    var caught = false;
+    let caught = false;
 
     cloudscraper(uri)
       .catch(function () {
@@ -102,7 +102,7 @@ describe('Cloudscraper', function () {
       res.endAbruptly();
     });
 
-    var caught = false;
+    let caught = false;
 
     cloudscraper(uri)
       .then(function () {
diff --git a/test/test-sandbox.js b/test/test-sandbox.js
new file mode 100644
index 0000000..faed30a
--- /dev/null
+++ b/test/test-sandbox.js
@@ -0,0 +1,107 @@
+/* eslint-disable no-unused-expressions */
+/* eslint-env node, mocha */
+'use strict';
+
+const sandbox = require('../lib/sandbox');
+const expect  = require('chai').expect;
+
+describe('Sandbox (lib)', function () {
+  it('should export Context', function () {
+    expect(sandbox.Context).to.be.a('function');
+  });
+
+  it('should export eval', function () {
+    expect(sandbox.eval).to.be.a('function');
+    expect(sandbox.eval('0')).to.equal(0);
+    expect(sandbox.eval('true')).to.be.true;
+    expect(sandbox.eval('undefined')).to.equal(void 0);
+    expect(sandbox.eval('NaN')).to.be.a('number');
+    expect(String(sandbox.eval('NaN'))).to.equal('NaN');
+  });
+
+  it('new Context() should return an object', function () {
+    expect(new sandbox.Context()).to.be.an('object');
+  });
+
+  it('Context() should define atob', function () {
+    const ctx = new sandbox.Context();
+
+    expect(ctx.atob).to.be.a('function');
+    expect(ctx.atob('YWJj')).to.equal('abc');
+    expect(sandbox.eval('atob("YWJj")', ctx)).to.equal('abc');
+  });
+
+  it('Context() should define location.reload', function () {
+    const ctx = new sandbox.Context();
+
+    expect(ctx.location).to.be.an('object');
+    expect(ctx.location.reload).to.be.an('function');
+
+    // This is a noop
+    ctx.location.reload();
+    expect(sandbox.eval('location.reload()', ctx)).to.equal(void 0);
+  });
+
+  it('Context() should define document.createElement', function () {
+    let ctx = new sandbox.Context();
+    let pseudoElement = { firstChild: { href: 'http:///' } };
+
+    expect(ctx.document).to.be.an('object');
+
+    expect(ctx.document.createElement).to.be.an('function');
+    expect(ctx.document.createElement('a')).eql(pseudoElement);
+    expect(sandbox.eval('document.createElement("a")', ctx)).to.eql(pseudoElement);
+
+    ctx = new sandbox.Context({ hostname: 'test.com' });
+    pseudoElement = { firstChild: { href: 'http://test.com/' } };
+
+    expect(ctx.document.createElement('a')).eql(pseudoElement);
+    expect(sandbox.eval('document.createElement("a")', ctx)).to.eql(pseudoElement);
+  });
+
+  it('Context() should define document.geElementById', function () {
+    let ctx = new sandbox.Context();
+
+    expect(ctx.document).to.be.an('object');
+
+    expect(ctx.document.getElementById).to.be.an('function');
+    expect(ctx.document.getElementById()).to.be.null;
+    expect(sandbox.eval('document.getElementById()', ctx)).to.be.null;
+    expect(ctx.document.getElementById('foobar')).to.be.null;
+    expect(sandbox.eval('document.getElementById("foobar")', ctx)).to.be.null;
+
+    // Double quotes
+    ctx = new sandbox.Context({ body: '<div id="test">foobar</div>' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: 'foobar' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: 'foobar' });
+
+    // Single quotes
+    ctx = new sandbox.Context({ body: '<div id=\'test\'>foobar</div>' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: 'foobar' });
+    expect(sandbox.eval('document.getElementById(\'test\')', ctx)).eql({ innerHTML: 'foobar' });
+
+    // Empty
+    ctx = new sandbox.Context({ body: '<div id="test"></div>' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: '' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: '' });
+
+    // Space agnostic tests
+    ctx = new sandbox.Context({ body: '<div id="test">\nabc\n\n</div>' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: '\nabc\n\n' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: '\nabc\n\n' });
+
+    ctx = new sandbox.Context({ body: '<div     id=\'test\'       > abc  </div>' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: ' abc  ' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: ' abc  ' });
+
+    ctx = new sandbox.Context({ body: 'foo="bar"  id=\'test\'  a=b  > abc  <' });
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: ' abc  ' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: ' abc  ' });
+
+    // Cache test
+    ctx = new sandbox.Context({ body: '<div id="test">foobar</div>' });
+    ctx.document.getElementById('test').innerHTML = 'foo';
+    expect(ctx.document.getElementById('test')).eql({ innerHTML: 'foo' });
+    expect(sandbox.eval('document.getElementById("test")', ctx)).eql({ innerHTML: 'foo' });
+  });
+});
diff --git a/test/test-timeout.js b/test/test-timeout.js
new file mode 100644
index 0000000..e69de29