A Chrome extension that lets you modify running apps without reloading them.





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This project is not actively maintained. Proceed at your own risk!


fb-flo is a Chrome extension that lets you modify running apps without reloading. It's easy to integrate with your build system, dev environment, and can be used with your favorite editor. Read more about it on https://facebook.github.io/fb-flo/


fb-flo is made up of a server and client component. This will guide through configuring your server for your project and installing the Chrome extension.

1. Configure fb-flo server

$ npm install fb-flo

fb-flo exports a single fb-flo function to start the server. Here is an example where you have your source JavaScript and CSS files in the root directory and your build step involves bundling both into a respective bundle.js, bundle.css.

var flo = require('fb-flo'),
    path = require('path'),
    fs = require('fs');

var server = flo(
    port: 8888,
    host: 'localhost',
    verbose: false,
    glob: [
       // All JS files in `sourceDirToWatch` and subdirectories
       // All CSS files in `sourceDirToWatch` and subdirectories
  function resolver(filepath, callback) {
    // 1. Call into your compiler / bundler.
    // 2. Assuming that `bundle.js` is your output file, update `bundle.js`
    //    and `bundle.css` when a JS or CSS file changes.
      resourceURL: 'bundle' + path.extname(filepath),
      // any string-ish value is acceptable. i.e. strings, Buffers etc.
      contents: fs.readFileSync(filepath),
      update: function(_window, _resourceURL) {
        // this function is executed in the browser, immediately after the resource has been updated with new content
        // perform additional steps here to reinitialize your application so it would take advantage of the new resource
        console.log("Resource " + _resourceURL + " has just been updated with new content");

flo takes the following arguments.

  • sourceDirToWatch: absolute or relative path to the directory to watch that contains the source code that will be built.
  • options hash of options:
    • port port to start the server on (defaults to 8888).
    • host to listen on.
    • verbose true or false value indicating if flo should be noisy.
    • glob a glob string or array of globs to match against the files to watch.
    • useWatchman when watching a large number of folders or where watching is buggy you can use (watchman)[https://facebook.github.io/watchman/].
    • useFilePolling some platforms that do not support native file watching, you can force the file watcher to work in polling mode.
    • pollingInterval if in polling mode (useFilePolling) then you can set the interval (in milliseconds) at which to poll for file changes.
    • watchDotFiles dot files are not watched by default.
    • disableWatcher set to true to disable starting a watcher (if you want to integrate with your own watcher)
  • resolver a function to map between files and resources.

The resolver callback is called with two arguments:

  • filepath path to the file that changed relative to the watched directory.
  • callback called to update a resource file in the browser. Should be called with an object with the following properties:
    • resourceURL used as the resource identifier in the browser.
    • contents any string-ish value representing the source of the updated file. i.e. strings, Buffers etc.
    • reload (optional) forces a full page reload. Use this if you're sure the changed code cannot be hotswapped.
    • match (optional, defaults to: indexOf) identifies the matching function to be performed on the resource URL in the browser. Could be one of the following:
      • "equal" test the updated resource resourceURL against existing browser resources using an equality check.
      • "indexOf" use String.prototype.indexOf check
      • /regexp/ a regexp object to exec.
    • update (optional) a function that will be executed in the browser, immediately after the resource has been updated. This can be used to run custom code that updates your application. It receives the window and the resourceURL as parameters. This function will be stringified so it could be sent to the client. Make sure you don't use any variables defined outside this function, as they won't be available, and you will get an error.

2. Install the Chrome Extension

Grab the fb-flo Chrome extension. This will add a new tab in your Chrome DevTools called 'flo'.

3. Activate fb-flo

To activate fb-flo from the browser:

  • Open Chrome DevTools.
  • Click on the new 'fb-flo' pane.
  • Click on 'Activate for this site'

See screenshot:

As an alternative to the update function, after any resource is updated, the fb-flo-reload event will be triggered on the window. The event's data will contain the url and contents that were provided to the callback function on the flo-server. The difference between the the update function and the fb-flo-reload event is that the first one is defined on the server and executed in the client, while the later is defined on the client and executed there as well. It is preferred to use the update function, since you won't load your app with code specific to live-editing. Example:

window.addEventListener('fb-flo-reload', function(ev) {
    // perform additional steps here to reinitialize your application so it would take advantage of the new resource
    console.log("Resource " + ev.data.url + " has just been replaced with this new content: " + ev.data.contents);


Say you have a Makefile program that builds your JavaScript and CSS into build/build.js and build/build.css respectively, this how you'd configure your fb-flo server:

var flo = require('fb-flo'),
    fs = require('fs'),
    path = require('path'),
    exec = require('child_process').exec;

var server = flo('./lib/', {
  port: 8888,
  dir: './lib/',
  glob: ['./lib/**/*.js', './lib/**/*.css']
}, resolver);

server.once('ready', function() {

function resolver(filepath, callback) {
    exec('make', function (err) {
      if (err) throw err;
        resourceURL: 'build/build' + path.extname(filepath),
        contents: fs.readFileSync('build/build' + path.extname(filepath)).toString()

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