Support node scripts in BROWSER

Modify OpenBrowser.js to run node scripts specified with the BROWSER environment
variable . If the value of the BROWSER environment variable ends with '.js' a
child process is spawned to execute the script with node.js. Any
arguments passed to npm start are also passed to this script, as well as
the url where the app is served.
The command executed in the child process is:

node <pathToScript> [OPTIONS] <url>

Update User Guide.

packages/react-dev-utils/openBrowser.js

@@ -10,22 +10,61 @@
 'use strict';
 
 var execSync = require('child_process').execSync;
+var spawn = require('cross-spawn');
 var opn = require('opn');
 
 // https://github.com/sindresorhus/opn#app
 var OSX_CHROME = 'google chrome';
 
-function openBrowser(url) {
+
+/**
+ * Use an enum for the possible uses of the BROWSER environment variable
+ * 'browser': open a browser specified by the user. if value is not specified,
+ * use the system's default browser.
+ * 'browser': open a browser.
+ * 'none': Don't open anything.
+ * 'script': run a node.js script.
+ *
+ */
+const BROWSER_TYPES = Object.freeze({
+  'browser': 0,
+  'none': 1,
+  'script': 2,
+})
+
+/**
+ * Returns an object with the BROWSER environment variable's value and type
+ */
+function getBrowserEnv() {
   // Attempt to honor this environment variable.
   // It is specific to the operating system.
   // See https://github.com/sindresorhus/opn#app for documentation.
-  var browser = process.env.BROWSER;
+  const browserStr = process.env.BROWSER;
+  const result = { value: browserStr }
+  if (!browserStr)
+    result.type = BROWSER_TYPES.browser;
+  else if (browserStr && browserStr.endsWith('.js'))
+    result.type = BROWSER_TYPES.script;
+  else if (browserStr.toLowerCase() === 'none')
+    result.type = BROWSER_TYPES.none;
+  else
+    result.type = BROWSER_TYPES.browser;
+  return result
+}
 
-  // Special case: BROWSER="none" will prevent opening completely.
-  if (browser === 'none') {
-    return false;
-  }
+/**
+ * spawns a new child process to execute a node.js script
+ */
+function executeNodeScript(scriptPath, url) {
+  const extraArgs = process.argv.slice(2);
+  const args = [scriptPath]
+                .concat(extraArgs) // add any extra flags
+                .concat(url); // add url last
+  spawn('node', args);
+  return true;
+}
 
+function startBrowserProcess(browser, url) {
   // If we're on OS X, the user hasn't specifically
   // requested a different browser, we can try opening
   // Chrome with AppleScript. This lets us reuse an
@@ -67,4 +106,27 @@ function openBrowser(url) {
   }
 }
 
+/**
+ * Reads the BROWSER evironment variable and decides what to do with it. Returns
+ * true if it opened a browser or ran a node.js script, otherwise false.
+ */
+function openBrowser(url) {
+  const browser = getBrowserEnv();
+
+  switch (browser.type) {
+    case BROWSER_TYPES.none:
+      // Special case: BROWSER="none" will prevent opening completely.
+      return false;
+    case BROWSER_TYPES.script:
+      return executeNodeScript(browser.value, url);
+    case BROWSER_TYPES.browser: {
+      return startBrowserProcess(browser.value, url);
+    }
+    default: {
+      throw new Error('Unknown Browser Type: ' + browser.type);
+    }
+  }
+
+}
+
 module.exports = openBrowser;

packages/react-dev-utils/package.json

@@ -29,6 +29,7 @@
     "anser": "1.1.0",
     "babel-code-frame": "6.20.0",
     "chalk": "1.1.3",
+    "cross-spawn": "^4.0.0",
     "escape-string-regexp": "1.0.5",
     "filesize": "3.3.0",
     "gzip-size": "3.0.0",

packages/react-scripts/template/README.md

@@ -1551,7 +1551,7 @@ You can adjust various development and production settings by setting environmen
 
 Variable | Development | Production | Usage
 :--- | :---: | :---: | :---
-BROWSER | :white_check_mark: | :x: | By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a [browser](https://github.com/sindresorhus/opn#app) to override this behavior, or set it to `none` to disable it completely.
+BROWSER | :white_check_mark: | :x: | By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a [browser](https://github.com/sindresorhus/opn#app) to override this behavior, or set it to `none` to disable it completely. If you need to customize the way the browser is launched, you can specify a node script instead. Any arguments passed to `npm start` will also be passed to this script, and the url where your app is served will be the last argument. Your script's file name must have the `.js` extension.
 HOST | :white_check_mark: | :x: | By default, the development web server binds to `localhost`. You may use this variable to specify a different host.
 PORT | :white_check_mark: | :x: | By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port.
 HTTPS | :white_check_mark: | :x: | When set to `true`, Create React App will run the development server in `https` mode.