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LoadJS

LoadJS is a tiny async loader for modern browsers (961 bytes).

CDNJS

Introduction

LoadJS is a tiny async loading library for modern browsers (IE9+). It has a simple yet powerful dependency management system that lets you fetch JavaScript, CSS and image files in parallel and execute code after the dependencies have been met. The recommended way to use LoadJS is to include the minified source code of loadjs.js in your <html> (possibly in the <head> tag) and then use the loadjs global to manage JavaScript dependencies after pageload.

LoadJS is based on the excellent $script library by Dustin Diaz. We kept the behavior of the library the same but we re-wrote the code from scratch to add support for success/error callbacks and to optimize the library for modern browsers. LoadJS is 961 bytes (minified + gzipped).

Here's an example of what you can do with LoadJS:

<script src="//unpkg.com/loadjs@latest/dist/loadjs.min.js"></script>
<script>
  // define a dependency bundle and execute code when it loads
  loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar');

  loadjs.ready('foobar', function() {
    /* foo.js & bar.js loaded */
  });
</script>

You can also use more advanced syntax for more options:

<script src="//unpkg.com/loadjs@latest/dist/loadjs.min.js"></script>
<script>
  // define a dependency bundle with advanced options
  loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar', {
    before: function(path, scriptEl) { /* execute code before fetch */ },
    async: true,  // load files synchronously or asynchronously (default: true)
    numRetries: 3,  // see caveats about using numRetries with async:false (default: 0)
    returnPromise: false  // return Promise object (default: false)
  });

  loadjs.ready('foobar', {
    success: function() { /* foo.js & bar.js loaded */ },
    error: function(depsNotFound) { /* foobar bundle load failed */ },
  });
</script>

The latest version of LoadJS can be found in the dist/ directory in this repository:

It's also available from these public CDNs:

You can also use it as a CJS or AMD module:

$ npm install --save loadjs
var loadjs = require('loadjs');

loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar');

loadjs.ready('foobar', function() {
  /* foo.js & bar.js loaded */
});

Browser Support

  • IE9+ (async: false support only works in IE10+)
  • Opera 12+
  • Safari 5+
  • Chrome
  • Firefox
  • iOS 6+
  • Android 4.4+

LoadJS also detects script load failures from AdBlock Plus and Ghostery in:

  • Safari
  • Chrome

Note: LoadJS treats empty CSS files as load failures in IE9-11 and uses rel="preload" to load CSS files in Edge (to get around lack of support for onerror events on <link rel="stylesheet"> tags)

Documentation

  1. Load a single file

    loadjs('/path/to/foo.js', function() {
  /* foo.js loaded */
});

  2. Fetch files in parallel and load them asynchronously

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], function() {
  /* foo.js and bar.js loaded */
});

  3. Fetch JavaScript, CSS and image files

    loadjs(['/path/to/foo.css', '/path/to/bar.png', 'path/to/thunk.js'], function() {
  /* foo.css, bar.png and thunk.js loaded */
});

  4. Force treat file as CSS stylesheet

    loadjs(['css!/path/to/cssfile.custom'], function() {
  /* cssfile.custom loaded as stylesheet */
});

  5. Force treat file as image

    loadjs(['img!/path/to/image.custom'], function() {
  /* image.custom loaded */
});

  6. Load JavaScript files as modules with non-module fallbacks (in browsers without module support)

    loadjs(['module!/path/to/foo.js', 'nomodule!/path/to/bar.js'], function() {
    /* foo.js loaded with type="module" in browsers with module support, skipped silently in browsers without */
    /* bar.js loaded with type="text/javascript" in browsers without module support, skipped silently in browsers with */
});

  7. Add a bundle id

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar', function() {
  /* foo.js & bar.js loaded */
});

  8. Use .ready() to define bundles and callbacks separately

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar');

loadjs.ready('foobar', function() {
  /* foo.js & bar.js loaded */
});

  9. Use multiple bundles in .ready() dependency lists

    loadjs('/path/to/foo.js', 'foo');
loadjs(['/path/to/bar1.js', '/path/to/bar2.js'], 'bar');

loadjs.ready(['foo', 'bar'], function() {
  /* foo.js & bar1.js & bar2.js loaded */
});

  10. Chain .ready() together

    loadjs('/path/to/foo.js', 'foo');
loadjs('/path/to/bar.js', 'bar');

loadjs
  .ready('foo', function() {
    /* foo.js loaded */
  })
  .ready('bar', function() {
    /* bar.js loaded */
  });

  11. Use Promises to register callbacks

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], {returnPromise: true})
  .then(function() { /* foo.js & bar.js loaded */ })
  .catch(function(pathsNotFound) { /* at least one didn't load */ });

  12. Check if bundle has already been defined

    if (!loadjs.isDefined('foobar')) {
  loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar', function() {
    /* foo.js & bar.js loaded */
  });
}

  13. Fetch files in parallel and load them in series

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], {
  success: function() { /* foo.js and bar.js loaded in series */ },
  async: false
});

  14. Add an error callback

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar', {
  success: function() { /* foo.js & bar.js loaded */ },
  error: function(pathsNotFound) { /* at least one path didn't load */ }
});

  15. Retry files before calling the error callback

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], 'foobar', {
  success: function() { /* foo.js & bar.js loaded */ },
  error: function(pathsNotFound) { /* at least one path didn't load */ },
  numRetries: 3
});

// NOTE: Using `numRetries` with `async: false` can cause files to load out-of-sync on retries

  16. Execute a callback before script tags are embedded

    loadjs(['/path/to/foo.js', '/path/to/bar.js'], {
  success: function() {},
  error: function(pathsNotFound) {},
  before: function(path, scriptEl) {
    /* called for each script node before being embedded */
    if (path === '/path/to/foo.js') scriptEl.crossOrigin = true;
  }
});

  17. Bypass LoadJS default DOM insertion mechanism (DOM <head>)

    loadjs(['/path/to/foo.js'], {
  success: function() {},
  error: function(pathsNotFound) {},
  before: function(path, scriptEl) {
    document.body.appendChild(scriptEl);
  
    /* return `false` to bypass default DOM insertion mechanism */
    return false;
  }
});

  18. Use bundle ids in error callback

    loadjs('/path/to/foo.js', 'foo');
loadjs('/path/to/bar.js', 'bar');
loadjs(['/path/to/thunkor.js', '/path/to/thunky.js'], 'thunk');

// wait for multiple depdendencies
loadjs.ready(['foo', 'bar', 'thunk'], {
  success: function() {
    // foo.js & bar.js & thunkor.js & thunky.js loaded
  },
  error: function(depsNotFound) {
    if (depsNotFound.indexOf('foo') > -1) {};  // foo failed
    if (depsNotFound.indexOf('bar') > -1) {};  // bar failed
    if (depsNotFound.indexOf('thunk') > -1) {};  // thunk failed
  }
});

  19. Use .done() for more control

    loadjs.ready(['dependency1', 'dependency2'], function() {
  /* run code after dependencies have been met */
});

function fn1() {
  loadjs.done('dependency1');
}

function fn2() {
  loadjs.done('dependency2');
}

  20. Reset dependency trackers

    loadjs.reset();

  21. Implement a require-like dependency manager

    var bundles = {
  'bundleA': ['/file1.js', '/file2.js'],
  'bundleB': ['/file3.js', '/file4.js']
};

function require(bundleIds, callbackFn) {
  bundleIds.forEach(function(bundleId) {
    if (!loadjs.isDefined(bundleId)) loadjs(bundles[bundleId], bundleId);
  });
  loadjs.ready(bundleIds, callbackFn);
}

require(['bundleA'], function() { /* bundleA loaded */ });
require(['bundleB'], function() { /* bundleB loaded */ });
require(['bundleA', 'bundleB'], function() { /* bundleA and bundleB loaded */ });

Directory structure

loadjs/
├── dist
│   ├── loadjs.js
│   ├── loadjs.min.js
│   └── loadjs.umd.js
├── examples
├── gulpfile.js
├── LICENSE.txt
├── package.json
├── README.md
├── src
│   └── loadjs.js
├── test
└── umd-templates

Development Quickstart

  1. Install dependencies

  2. Clone repository

    $ git clone git@github.com:kubetail-org/loadjs.git
$ cd loadjs

  3. Install node dependencies using npm

    $ npm install

  4. Build examples

    $ npm run build-examples

    To view the examples you can use any static file server. To use the nodejs http-server module:

    $ npm install http-server
$ npm run http-server -- -p 3000

    Then visit http://localhost:3000/examples

  5. Build distribution files

    $ npm run build-dist

    The files will be located in the dist directory.

  6. Run tests

    To run the browser tests first build the loadjs library:

    $ npm run build-tests

    Then visit http://localhost:3000/test

  7. Build all files

    $ npm run build-all