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JavaScript SDK Design Guide

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This guide provides an introduction to develop a JavaScript SDKs.

The best one sentence to describe an SDK is: "The SDK is the connection bridging the gap between users and the (browser) machine."

At the end of this guide, the SDK will be capable to run in browsers, desktop, mobile web and various other platforms capable of running JavaScript.

The target audience of this writeup exludes non-browser environments such as hardware, embedded and Node.js for now. However, future materials will be added to cover those areas.

It is possible to suggest improvements by editing, dropping suggestions on the issue list. I owe you a beer 🍻



What is SDK

This question is pretty ubiquitous, but here it is again.

"Short for software development kit, a programming package that enables a programmer to develop applications for a specific platform. Typically an SDK includes one or more APIs, programming tools, and documentation." - webopedia

Design Philosophy

Depending on the purpose of SDK's service and usage — common shared traits are, but not limited to be native, short, fast, clean, readable and testable.

The widely adopted good practice, is to write SDK with vanilla JavaScript. Languages compiling to JavaScript such as LiveScript, CoffeeScript, TypeScript and others are not recommended.

It is also recommended not to use libraries suchs as jQuery in SDK development. The exception is of course when it is really important. There are also other jQuery-like libraries, zepto.js etc to choose from, for the DOM manipulation purposes.

In event of HTTP ajax request requirements — there are native equivalent such as window.fetch. It is light-weight, supported in ever growing platforms.

Backward compatibility is paramount. Every new SDK version released should be enabled with support of previous older versions. Likewise, current version should be designed to support future SDK versions. This is referred to as Forward compatibility.

Moreover, a good Documentation, well commented code, a healthy unit test coverage, as well as end-to-end (user) scenario are key to the success of SDK.


Based on the book Third-Party JavaScript

Three use cases worth to consider while designing a JavaScript SDK:

  1. Embedded widgets - Small interactive applications embedded on the publisher's web page (Disqus, Google Maps, Facebook Widget)
  2. Analytics and metrics - For gathering intelligence about visitors and how they interact with the publisher's website (GA, Flurry, Mixpanel)
  3. Web service API wrappers - For developing client-side applications that communicate with external web services. (Facebook Graph API)

Suggest a case in which the use of an SDK in JavaScript environment is deemed important.

Include the SDK

To include the SDK in user-facing environment, It is a good practice use Asynchronous Syntax to load the scripts.

This helps to optimize the user experience on the website that are using the SDK. This approach reduce chances of the SDK library interfering with the hosting website.

Asynchronous Syntax

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = 'http://<DOMAIN>.com/sdk.js';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

The async syntax is used when targetting on the modern browsers.

<script async src="http://<DOMAIN>.com/sdk.js"></script>

Traditional Syntax

<script type="text/javascript" src="http://<DOMAIN>.com/sdk.js"></script>


Here's the simple graph to show the differentiate between Asynchronous and Traditional Syntax.





Asynchronous and deferred JavaScript execution explained
It is good practice to avoid, or minimize, the use of blocking JavaScript, especially external scripts that must be fetched before they can be executed. Scripts that are necessary to render page content can be inlined to avoid extra network requests, however the inlined content needs to be small and must execute quickly(non-blocking fashion) to deliver good performance. Scripts that are not critical to initial render should be made asynchronous or deferred until after the first render.

Problem of Asynchronous

When using Asynchronous approach, It is ill advised to execute SDK initialization function before all libraries are loaded, parsed and executed in hosting page.

Considering the following snippet as a visual example to the previous statement:

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = 'http://<DOMAIN>.com/sdk.js';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName('some arguments');

The end result of such initialization will lead to bugs. The SDKName() function, undefined at this point, executes before it becomes available in the environment's global variable. The script is not loaded yet.

To make it work, some tricks are necessary to make sure the script execute with success. The event will store in the SDKName.q queue array. The SDK should be able to handle and execute SDKName.q event and initialize the SDKName namespace.

The following snippet depicts the statement in previous paragraph.

  (function () {
    // add a queue event here
    SDKName = SDKName || function () {
      (SDKName.q = SDKName.q || []).push(arguments);
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = 'http://<DOMAIN>.com/sdk.js';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName('some arguments');

Or using [].push

  (function () {
    // add a queue event here
    SDKName = window.SDKName || (window.SDKName = []);
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = 'http://<DOMAIN>.com/sdk.js';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

  // execute your script immediately here
  SDKName.push(['some arguments']);


There are other different ways to include a script

Import in ES2015

  import "your-sdk";

Modular include a Script

There is full source code — and this awesome tutorial "Loading JavaScript Modules" may help for in depth understanding of concepts discussed above.

module('sdk.js',['sdk-track.js', 'sdk-beacon.js'],function(track, beacon) {
  // sdk definitions, split into local and global/exported definitions
  // local definitions
  // exports

// you should contain this "module" method
(function () {

  var modules = {}; // private record of module data

  // modules are functions with additional information
  function module(name,imports,mod) {

    // record module information
    window.console.log('found module '+name);
    modules[name] = {name:name, imports: imports, mod: mod};

    // trigger loading of import dependencies
    for (var imp in imports) loadModule(imports[imp]);

    // check whether this was the last module to be loaded
    // in a given dependency group

  // function loadModule
  // function loadedModule

  window.module = module;

SDK Versioning

It is not a good practice to use one of the following versioning styles:

  • brand-v<timestamp>.js
  • brand-v<datetime>.js
  • brand-v1-v2.js,

The reason behind this is that It becomes confusing to track the lastest version. Therefore, previous styling do not help developers who use the SDK.

It is however a good practice to use Semantic Versioning, a.k.a SemVer, when versioning SDKs. It worths to mention that SemVer has three main parts, each corresponding to importance of a release: "MAJOR.MINOR.PATCH". Version in v1.0.0 v1.5.0 v2.0.0 is easier to trace and track in changelog documentations, for instance.

Depending on service design, some of the ways SDK can be distributed(or tracked) by version are the following:

  • Using Query String path — http://<DOMAIN>.com/sdk.js?v=1.0.0
  • Using the Folder Naming — http://<DOMAIN>.com/v1.0.0/sdk.js
  • Using hostname (subdomain) — http://v1.<DOMAIN>.com/sdk.js

Depending on use case, there are other environmental dependant forms that are commonly advised to use:

  • In stable version http://<DOMAIN>.com/sdk-stable.js
  • In unstable version http://<DOMAIN>.com/sdk-unstable.js
  • In alpha version http://<DOMAIN>.com/sdk-alpha.js
  • In latest version http://<DOMAIN>.com/sdk-latest.js
  • In experimental version http://<DOMAIN>.com/sdk-experimental.js

Reading suggestion: Why use SemVer? on npm blog.

Changelog Document

It is hard to notice when an SDK has updates(or is upgraded) when no announcement has been issued. It is a good practice to write a changelog to document major, minor and even bug fix changes. Tracing changes in SDK APIs deliver good developer experience. - Keep a Changelog (Github Repo)

Each version should have:

[Added] for new features.
[Changed] for changes in existing functionality.
[Deprecated] for once-stable features removed in upcoming releases.
[Removed] for deprecated features removed in this release.
[Fixed] for any bug fixes.
[Security] to invite users to upgrade in case of vulnerabilities.

In addition, commit-message-emoji uses emoji to explain the commit's changes itself.


To avoid collision with other libraries, It is better to define no more than one global SDK namespace. The naming should also avoid using the commonly used words and catch-phrases as namespaces.

As a quick example, SDK playground can well use (function () { ... })() or ES6 Blocks { ... } to wrap all sources.

This is an increasingly common practice found in many popular JavaScript libraries such as (jQuery, Node.js, etc.). This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries. #

To avoid namespace collision

From Google Analytics, define the namespace by changing the value ga

(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),

From OpenX experience, support a parameter to request the namespace.

<script src="http://your_domain/sdk?namespace=yourcompany"></script>

Storage Mechanism


The domain scope of using cookie is quite complex while involving the subdomain and path.

For path=/, there is a cookie part first=value1 in domain, and another cookie second=value2 in domain

There is a cookie first=value1 in domain, cookie second=value2 in domain path and cookie third=value3 in domain,

Check Cookie Writable

Given a domain (Default as current hostname), check whether the cookie is writable.

var checkCookieWritable = function(domain) {
    try {
        // Create cookie
        document.cookie = 'cookietest=1' + (domain ? '; domain=' + domain : '');
        var ret = document.cookie.indexOf('cookietest=') != -1;
        // Delete cookie
        document.cookie = 'cookietest=1; expires=Thu, 01-Jan-1970 00:00:01 GMT' + (domain ? '; domain=' + domain : '');
        return ret;
    } catch (e) {
        return false;

Check Third Party Cookie Writable

It's impossible to check only using client side JavaScript, but a server can help to achive just that.

Write Read Remove Cookie Code

Snippet Code for write/read/remove cookie script.

var cookie = {
    write: function(name, value, days, domain, path) {
        var date = new Date();
        days = days || 730; // two years
        path = path || '/';
        date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
        var expires = '; expires=' + date.toGMTString();
        var cookieValue = name + '=' + value + expires + '; path=' + path;
        if (domain) {
            cookieValue += '; domain=' + domain;
        document.cookie = cookieValue;
    read: function(name) {
        var allCookie = '' + document.cookie;
        var index = allCookie.indexOf(name);
        if (name === undefined || name === '' || index === -1) return '';
        var ind1 = allCookie.indexOf(';', index);
        if (ind1 == -1) ind1 = allCookie.length;
        return unescape(allCookie.substring(index + name.length + 1, ind1));
    remove: function(name) {
        if ( {
            this.write(name, '', -1, '/');


It's important to know that in JavaScript It is not possible to write Session. That is the server responsibility. The server side team should implement Session management related use cases.

A page session lasts for as long as the browser is open and survives over page reloads and restores. Opening a page in a new tab or window will cause a new session to be initiated.


Stores data with no expiration date, storage limit is far larger (at least 5MB) and information is never transferred to the server.

It's good to know that each localStorage from http and https in the same domain aren't shared. Creating an iframe inside a website and using postMessage to pass the value to others.

Check LocalStorage Writable

window.localStorage is not support in every browser, SDK should check available before using it.

var testCanLocalStorage = function() {
   var mod = 'modernizr';
   try {
       localStorage.setItem(mod, mod);
       return true;
   } catch (e) {
       return false;

Session Storage

Stores data for one session (data is lost when the tab is closed).

Check SessionStorage Writable

var checkCanSessionStorage = function() {
  var mod = 'modernizr';
  try {
    sessionStorage.setItem(mod, mod);
    return true;
  } catch (e) {
    return false;


In client browser, there are events load unload on off bind .... Here's some polyfills for you to handle all different platforms.

Document Ready

Please do make sure that the entire page is finished loading(ready) before starting execution the SDK functions.

// handle IE8+
function ready (fn) {
    if (document.readyState != 'loading') {
    } else if (window.addEventListener) {
        // window.addEventListener('load', fn);
        window.addEventListener('DOMContentLoaded', fn);
    } else {
        window.attachEvent('onreadystatechange', function() {
            if (document.readyState != 'loading')

DOMContentLoaded - fired when the document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading

load event can be used to detect a fully-loaded page

Information from JS Tip -

element-ready from sindresorhus

Message Event

It's about the cross-origin communication between iframe and window, read the API documentation.

// in the iframe
parent.postMessage("Hello"); // string

// ==========================================

// in the iframe's parent
// Create IE + others compatible event handler
var eventMethod = window.addEventListener ? "addEventListener" : "attachEvent";
var eventer = window[eventMethod];
var messageEvent = eventMethod == "attachEvent" ? "onmessage" : "message";

// Listen to message from child window
eventer(messageEvent,function(e) {
  // e.origin , check the message origin
  console.log('parent received message!:  ',;

The post message data should be String, for more advanced used in json, use JSON String. Although the modern browser do support Structured Clone Algorithm on the parameter, but not the 100% browser.

Orientation Change

Detect device orientation change

window.addEventListener('orientationchange', fn);

Get Orientation Rotate Degree

window.orientation; // => 90, -90, 0

Screen portrait-primary, portrait-secondary, landscape-primary, landscape-secondary (Experimental)

var orientation = screen.orientation || screen.mozOrientation || screen.msOrientation;

Disable Scroll

In web page, use CSS style overflow: hidden, in some mobile web, this css doesn't work, use javascript event.

document.addEventListener('touchstart', function(e){ e.preventDefault(); }); // prevent scroll
// or
document.body.addEventListener('touchstart', function(e){ e.preventDefault(); }); // prevent scroll
// use move if you need some touch event
document.addEventListener('touchmove', function(e){ e.preventDefault(); }); // prevent scroll


The communication between our SDK and Server is using Ajax Request. Most commonly use-cases leverage jQuery's ajax http request to communicate with Server. The good news is that there is even a better solution to achieve that.

Image Beacon

Using the Image Beacon to ask the browser to perform a method GET Request to get an Image.

Ones should always remember to add timestamp (Cache Buster) to prevent caching in browser.

(new Image()).src = 'http://<DOMAIN>.com/collect?id=1111';

Some notice for GET Query String, there is the limit of length which is 2048(Basically It depends on different browser and server). The following trick helps to handle the case of exceeded length limit.

if (length > 2048) {
    // do Multiple Post (form)
} else {
    // do Image Beacon

There are well known problems using encodeURI or encodeURIComponent. However, It is better to understand how these two approaches work. Reading details below.

For the image load success/error callback

var img = new Image();
img.src = 'http://<DOMAIN>.com/collect?id=1111';
img.onload = successCallback;
img.onerror = errorCallback;

Single Post

it is possible to use the native form element method POST to send a key value.

var form = document.createElement('form');
var input = document.createElement('input'); = 'none';
form.setAttribute('method', 'POST');
form.setAttribute('action', 'http://<DOMAIN>.com/track'); = 'username';
input.value = 'attacker';



Multiple Post

The Service is often complex, especially when needing to send more data through POST method.

function requestWithoutAjax( url, params, method ){

    params = params || {};
    method = method || "post";

    // function to remove the iframe
    var removeIframe = function( iframe ){

    // make a iframe...
    var iframe = document.createElement('iframe'); = 'none';

    iframe.onload = function(){
        var iframeDoc = this.contentWindow.document;

        // Make a invisible form
        var form = iframeDoc.createElement('form');
        form.method = method;
        form.action = url;

        // pass the parameters
        for( var name in params ){
            var input = iframeDoc.createElement('input');
            input.type = 'hidden';
   = name;
            input.value = params[name];

        // remove the iframe
        setTimeout( function(){
        }, 500);

requestWithoutAjax('url/to', { id: 2, price: 2.5, lastname: 'Gamez'});


Iframe embedded in html can always be used to cover the use case of generating content within the page.

var iframe = document.createElement('iframe');
var body = document.getElementsByTagName('body')[0]; = 'none';
iframe.src = 'http://<DOMAIN>.com/page';
iframe.onreadystatechange = function () {
    if (iframe.readyState !== 'complete') {
iframe.onload = loadCallback;


Remove extra margin from INSIDE an iframe

<iframe src="..."

Putting html content into an iframe

<iframe id="iframe"></iframe>

  var html_string= "content <script>alert(location.href);</script>";
  document.getElementById('iframe').src = "data:text/html;charset=utf-8," + escape(html_string);
  // alert data:text/html;charset=utf-8.....
  // access cookie get ERROR

  var doc = document.getElementById('iframe').contentWindow.document;;
  // alert "top window url"

  var iframe = document.createElement('iframe');
  iframe.src = 'javascript:;\'' + encodeURI('<html><body><script>alert(location.href);</body></html>') + '\'';
  // iframe.src = 'javascript:;"' + encodeURI((html_tag).replace(/\"/g, '\\\"')) + '"';
  // alert "about:blank"

Script jsonp

This is the case that your server needs to response JavaScript code and let the client browser execute it. Just include the JS script link.

  (function () {
    var s = document.createElement('script');
    s.type = 'text/javascript';
    s.async = true;
    s.src = '/yourscript?some=parameter&callback=jsonpCallback';
    var x = document.getElementsByTagName('script')[0];
    x.parentNode.insertBefore(s, x);

To learn more about jsonp

  1. JSONP only works in GET HTTP request.
  2. JSONP lacks error handling, means you cannot detect case in response status code 404, 500 and so on.
  3. JSONP requests are always asynchronous.
  4. Beware of CSRF attack.
  5. Cross domain communication. Script response side (server-side) don't need to care about CORS.


Look at the documentation.

This method addresses the needs of analytics and diagnostics code that typically attempt to send data to a web server prior to the unloading of the document. Sending the data any sooner may result in a missed opportunity to gather data. However, ensuring that the data has been sent during the unloading of a document is something that has traditionally been difficult for developers.

Send POST beacon through the API. It's cool.

navigator.sendBeacon("/log", analyticsData);


Writing XMLHttpRequest is not a good idea. I assume that you don't want to waste time on battling with IE or other browsers. Here's some polyfills or code you can try

  1. window.fetch - A window.fetch JavaScript polyfill. (check also ky)
  2. got - Simplified HTTP/HTTPS requests
  3. microjs - list of ajax lib
  4. more

Fragment Identifier

Also known as hash mark #. Remember that request with hash mark at the end is not pass within http request.

For example, you are in the page

// Sending a request with a parameter url which contains current url
(new Image()).src = '';

// actual request will be without #
(new Image()).src = '';

// Solved, encodeURIComponent(url);
(new Image()).src = '' + encodeURIComponent('');

Maximum Number of Connection

Check the maximum number of browser's request connection. browserscope

max number of connection

Component of URI

It's important to know if the SDK needs to parse the location url.

                  /                    \
              userinfo                host                          resource
               __|___                ___|___                 __________|___________
              /      \              /       \               /                      \
         username  password     hostname    port     path & segment      query   fragment
           __|___   __|__    ______|______   |   __________|_________   ____|____   |
          /      \ /     \  /             \ / \ /                    \ /         \ / \
    \_/                     \ / \       \ /    \__________/ \     \__/
     |                       |   \       |           |       \      |
  scheme               subdomain  \     tld      directory    \   suffix
                                   \____/                      \___/
                                      |                          |
                                    domain                   filename

Parsing URI

Here's a simplest by using the native URL() Interface but it doesn't support all the browsers. It is also not a standard yet.

var parser = new URL('');
parser.hostname; // => ""

The DOM 's createElement('a') can be used in browsers that don't have the URL() Interface yet.

var parser = document.createElement('a');
parser.href = "";
parser.hostname; // => ""


Simulating Multiple Domains

To simulate multiple domains, there is no need to register different domain names. Editing operating system's hosts file can do the trick.

$ sudo vim /etc/hosts

Add the following entries

# refer to localhost

Every website URL becomes accessible via and

Developer Tools

Browsers come with a debugging tools specific to every vendor. Obviously, these tools can be used to debug SDK JavaScript code - Chrome Developer Tools Safari Developer Tools Firebug. Developer tools also short for DevTools.

The DevTools provide web developers deep access into the internals of the browser and their web application. Use the DevTools to efficiently track down layout issues, set JavaScript breakpoints, and get insights for code optimization.

Console Logs

For testing expected output text and other general debugging, Console Logs can be used throught the browser API console.log(). There are various typeways to format and output messages. There is more on this issue discussed on this link: Console API.

screen shot 2015-06-15 at 3 50 23 pm

Debugging Proxy

Debugging proxy gives a hand on testing SDK in development. Some of the areas covered are:

  • Debugging traffic
  • modify cookies
  • Inspecting headers
  • Verifying the cache
  • Editing http request/response
  • SSL Proxying
  • Debugging Ajax and more.

Here's some software you can try


BrowserSync makes it easy to tweak and test faster by synchronizing file changes and interactions across multiple devices. It’s wicked-fast and totally free.

It really helps a lot to test SDK across mutliple devices. Totally worths a try =)

Debugging Node.js Apps

To debug SDK scripts in Chrome Developer Tools. (Node.js v6.3.0+ required)

  $ node --inspect-brk [script.js]

Tips and Tricks


Sometimes, including all the SDK source code is not required in some use cases. That is the case of a simple 1x1 pixel request -- For example: make a request when someone lands on thank you(last) page. In such a scenario, the developer may include an image file with a the (url) link, as explained in the following snippet.

<img height="1" width="1" alt="" style="display:none" src="" />

Page Visibility API

Sometimes, the SDK wants to detect if a user has a particular page in focus. These polyfills visibly.js and visibilityjs may help achieve just that.

Document Referrer

The document.referrer can be used to get the url of current or previous page. It is however advised to remember that this referrer is "Browser Referrer" not the "Human Known Referrer". The case where a user clicks the browser back button, for example pageA -> pageB -> pageC -> (back button) pageB, current pageB's referrer is pageA, not pageC.

Console Logs Polyfill

The following is not a special polyfill. It just make sure that calling console.log API doesn't throw error event to client side.

if (typeof console === "undefined") {
    var f = function() {};
    console = {
        log: f,
        debug: f,
        error: f,
        info: f

EncodeURI or EncodeURIComponent

Understand the difference between escape() encodeURI() encodeURIComponent() here.

It worth mentioning that using encodeURI() and encodeURIComponent() has exactly 11 characters difference. These characters are: # $ & + , / : ; = ? @ more discussion.

encodeuri or encodeuricomponent


As the title said, you might not need jquery. It's really useful if you are looking for some utilities code - AJAX EFFECTS, ELEMENTS, EVENTS, UTILS

You Don't Need jQuery

Free yourself from the chains of jQuery by embracing and understanding the modern Web API and discovering various directed libraries to help you fill in the gaps.

Useful Tips

  1. Selecting Elements
  2. DOM Manipulation

Load Script with Callback

It's similar to asynchrnous script loading with addition callback event

function loadScript(url, callback) {
  var script = document.createElement('script');
  script.async = true;
  script.src = url;

  var entry = document.getElementsByTagName('script')[0];
  entry.parentNode.insertBefore(script, entry);

  script.onload = script.onreadystatechange = function () {
    var rdyState = script.readyState;

    if (!rdyState || /complete|loaded/.test(script.readyState)) {

      // detach the event handler to avoid memory leaks in IE (
      script.onload = null;
      script.onreadystatechange = null;

Once Function

Implementation of the function once

Quite often, there are functions that are needed only want to run once. Oftentimes these functions are in the form of event listeners which may be difficult to manage. Of course if they were easy to manage, it is advised to just remove the listeners. The following is the JavaScript function to make that possible!

// Copy from DWB
function once(fn, context) {
	var result;

	return function() {
		if(fn) {
			result = fn.apply(context || this, arguments);
			fn = null;

		return result;

// Usage
var canOnlyFireOnce = once(function() {

canOnlyFireOnce(); // "Fired!"
canOnlyFireOnce(); // nada

Pixel Ratio Density

To better understand terms such as pixel, ratio, density, dimension are while developing mobile web -- the following links can provide more insights:

Get Style Value

Get inline-style value

<span id="black" style="color: black"> This is black color span </span>
    document.getElementById('black').style.color; // => black

Get Real style value

#black {
    color: red !important;

<span id="black" style="color: black"> This is black color span </span>

    document.getElementById('black').style.color; // => black

    // real
    var black = document.getElementById('black');
    window.getComputedStyle(black, null).getPropertyValue('color'); // => rgb(255, 0, 0)


Check if Element in Viewport

There is more here.

function isElementInViewport (el) {

    //special bonus for those using jQuery
    if (typeof jQuery === "function" && el instanceof jQuery) {
        el = el[0];

    var rect = el.getBoundingClientRect();

    return ( >= 0 &&
        rect.left >= 0 &&
        rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /*or $(window).height() */
        rect.right <= (window.innerWidth || document.documentElement.clientWidth) /*or $(window).width() */

Check if Element is Visible

var isVisible = function(b) {
    var a = window.getComputedStyle(b);
    return 0 === a.getPropertyValue("opacity") || "none" === a.getPropertyValue("display") || "hidden" === a.getPropertyValue("visibility") || 0 === parseInt(, 10) || "none" === || "hidden" === ? false : true;

var element = document.getElementById('box');
isVisible(element); // => false or true

Get Viewport Size

var getViewportSize = function() {
    try {
        var doc = top.document.documentElement
          , g = (e = top.document.body) && top.document.clientWidth && top.document.clientHeight;
    } catch (e) {
        var doc = document.documentElement
          , g = (e = document.body) && document.clientWidth && document.clientHeight;
    var vp = [];
    doc && doc.clientWidth && doc.clientHeight && ("CSS1Compat" === document.compatMode || !g) ? vp = [doc.clientWidth, doc.clientHeight] : g && (vp = [doc.clientWidth, doc.clientHeight]);
    return vp;

// return as array [viewport_width, viewport_height]

User Tracking

Assuming that an Evil Advertisement Company wants to track a user, evil may well generate a personalized unique hash by using fingerprinting. However, ethical company uses cookies and offers Opt-out solution.

Opt Out

DIGITAL ADVERTISING ALLIANCE, POWERED BY YOURADCHOICES provides a tool that helps anyone to opt out from all the participating companies.


Misspelling Of Referrer

Fun fact about why the HTTP Request Header having the field name referer not referrer

According to the Wikipedia

The misspelling of referrer originated in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the field into the HTTP specification. The misspelling was set in stone by the time of its incorporation into the Request for Comments standards document RFC 1945; document co-author Roy Fielding has remarked that neither "referrer" nor the misspelling "referer" were recognized by the standard Unix spell checker of the period. "Referer" has since become a widely used spelling in the industry when discussing HTTP referrers; usage of the misspelling is not universal, though, as the correct spelling "referrer" is used in some web specifications such as the Document Object Model.


This guide provides templates and boilerplates to building an SDK.

Book/Nice to Read

  1. Third-Party JavaScript
  2. JQuery Plugin
  3. LightningJS


  1. What is Software Development Kit
  2. A window.fetch JavaScript polyfill.
  3. POST Request
  4. Semantic VersioningVersioning 2.0.0
  5. HTTP API design guide extracted from work on the Heroku Platform API
  6. Understanding URIs
  7. URI Parsing with JavaScript
  8. Modernizr: the feature detection library for HTML5/CSS3
  9. HTML5 Web Storage
  10. Check if third-party cookies are enabled
  11. Introduction to Analytics.js - Universal Analytics Web Tracking
  12. Facebook Conversion Tracking Pixel
  13. What is the maximum length of a URL
  15. What is a Polyfill?
  16. Asynchronous and deferred JavaScript execution explained
  17. generate random UUIDs
  18. DOMContentLoaded and Load Event
  19. Must See JavaScript Dev Tools That Put Other Dev Tools to Shame
  20. Learn the art of writing quality READMEs.
  21. The Deep Dive - JavaScript Widgets ~ How to design, build and deploy a JavaScript Widget that runs on other people’s websites

(inspired by http-api-design)


Thanks goes to these wonderful people (emoji key):

Huei Tan
Huei Tan

Pascal Maniraho
Pascal Maniraho



This project follows the all-contributors specification. Contributions of any kind welcome!

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