A JS library for the browser that automatically skips the initial API call(s) if the data is already available.

bower install apiprefetch




A library for the browser that automatically skips the initial API call that JS makes if the data is already available.

It monkey-patches the browser's default XMLHttpRequest constructor, so it should "just work" with any framework, including: jQuery, AngularJS, EmberJS, BackboneJS, etc.

Build Status


bower install apiprefetch


npm install apiprefetch


Load apiPrefetch.js before you load any other JS code, especially any that would access XMLHttpRequest.

Configure your server to preload the results of an initial API get request into a global object called apiPrefetchData with the keys equal to the endpoint path.

The value for each key should be a rendered/parsed JSON object (or a string if it's not JSON), but be sure to escape forward slashes (see security note below).

Voila, now when you go to make the initial API request, if the data's already there, it's returned without needing to make another round-trip to the server.

After the initial API request, all future requests to the same url path will actually go out and hit your API as usual.


      window.apiPrefetchData = {
        "/api/title": {"title":"Game of Thrones \/ A Song of Ice and Fire"},
        "/api/description": {"description":"Winter is Coming…"}
    <div id="title"></div>
    <div id="description"></div>
    <script src="apiPrefetch.js"></script>
    <script src="jquery.js"></script>
      // First call gets data rendered in the script block above
      $.getJSON('/api/title').done(function (res) {
        // Gets called on next tick, seems instantaneous to the user

        // Makes an actual API call to the server
        $.getJSON('/api/title').done(function (res) {
          alert("The title from the server: " + res.title);

      // Second call also gets pre-rendered data
      $.getJSON('/api/description').done(function (res) {

An important note on security

Since you will likely be rendering data from users, you need to protect against XSS attacks.

To do so, escape all / characters, e.g. \/. This prevents users from injecting JS code by closing the script tag the data is rendered into.


If you plan to use the browser's raw XMLHttpRequest class, be aware of the following:

  • In order for the library to work its magic, the xhr object it generates currently only has open and send methods.
  • Callbacks should be assigned to the xhr's onload property. Support for addEventListener and onreadystatechange isn't available yet, and likely won't be needed.
  • It does work with responseType, but only JSON. If you set xhr.responseType to "json", then xhr.response will be a parsed JSON object, not a JSON string.


Isn't this a huge hack?

Couldn't you say the same for AJAX itself? The X stands for XML and you're fetching JSON. You're already making the browser do things it wasn't originally intended to do :)


Created by Jon Abrams