Modern JavaScript libraries: the isomorphic way

Algolia’s DNA is really about performance. We want our search engine to answer relevant results as fast as possible.

To achieve the best end-to-end performance we’ve decided to go with JavaScript since the total beginning of Algolia. Our end-users search using our REST API directly from their browser – with JavaScript – without going through the websites’ backends.

Our JavaScript & Node.js API clients were implemented 2 years ago and were now lacking of all modern best practices:

  • not following the error-first or callback-last conventions;
  • inconsistent API between the Node.js and the browser implementations;
  • no Promise support;
  • Node.js module named algolia-search, browser module named algoliasearch;
  • cannot use the same module in Node.js or the browser (obviously);
  • browser module could not be used with browserify or webpack. It was exporting multiple properties directly in the window object.

This blog post is a summary of the three main challenges we faced while modernizing our JavaScript client.


Now the good news: we have a new isomorphic JavaScript API client.

Isomorphic JavaScript apps are JavaScript applications that can run both client-side and server-side.
The backend and frontend share the same code.

Here are the main features of this new API client:

If you were using our previous libraries, we have migration guides for both Node.js and the browser.

Challenge #1: testing

Before being able to merge the Node.js and browser module, we had to remember how the current code is working. An easy way to understand what a code is doing is to read the tests. Unfortunately, in the previous version of the library, we had only one test. One test was not enough to rewrite our library. Let’s go testing!

Unit? Integration?

When no tests are written on a library of ~1500+ LOC, what are the tests you should write first?

Unit testing would be too close to the implementation. As we are going to rewrite a lot of code later on, we better not go too far on this road right now.

Here’s the flow of our JavaScript library when doing a search:

  • initialize the library with algoliasearch()
  • call'something', callback)
  • browser issue an HTTP request
  • callback(err, content)

From a testing point of view, this can be summarized as:

  • input: method call
  • output: HTTP request

Integration testing for a JavaScript library doing HTTP calls is interesting but does not scale well.

Indeed, having to reach Algolia servers in each test would introduce a shared testing state amongst developers and continuous integration. It would also have a slow TDD feedback because of heavy network usage.

Our strategy for testing our JavaScript API client was to mock (do not run away right now) the XMLHttpRequest object. This allowed us to test our module as a black box, providing a good base for a complete rewrite later on.

This is not unit testing nor integration testing, but in between. We also planned in the coming weeks on doing a separate full integration testing suite that will go from the browser to our servers.

faux-jax to the rescue

Two serious candidates showed up to help in testing HTTP request based libraries

Unfortunately, none of them met all our requirements. Not to mention, the AlgoliaSearch JavaScript client had a really smart failover request strategy:

This seems complex but we really want to be available and compatible with every browser environment.

  • Nock works by mocking calls to the Node.js http module, but we directly use the XMLHttpRequest object.
  • Sinon.js was doing a good job but was lacking some XDomainRequest feature detections. Also it was really tied to Sinon.js.

As a result, we created algolia/faux-jax. It is now pretty stable and can mock XMLHttpRequest, XDomainRequest and even http module from Node.js. It means faux-jax is an isomorphic HTTP mock testing tool. It was not designed to be isomorphic. It was easy to add the Node.js support thanks to moll/node-mitm.

Testing stack

The testing stack is composed of:

The fun part is done, now onto the tedious one: writing tests.

Spliting tests cases

We divided our tests in two categories:

  • simple test cases: check that an API command will generate the corresponding HTTP call
  • advanced tests: timeouts, keep-alive, JSONP, request strategy, DNS fallback, ..

Simple test cases

Simple test cases were written as table driven tests:

It's a simple JavaScript file, exporting test cases as an array
It’s a simple JavaScript file, exporting test cases as an array

Creating a testing stack that understands theses test-cases was some work. But the reward was worth it: the TDD feedback loop is great. Adding a new feature is easy: fire editor, add test, implement annnnnd done.

Advanced tests

Complex test cases like JSONP fallback, timeouts and errors, were handled in separate, more advanced tests:

Our testing stack rely on substack/tape
Here we test that we are using JSONP when XHR fails

Testing workflow

To be able to run our tests we chose defunctzombie/zuul.

Local development

For local development, we have an npm test task that will:

  • launch the browser tests using phantomjs,
  • run the Node.js tests,
  • lint using eslint.

You can see the task in the package.json. Once run it looks like this:

640 passing assertions and counting!
640 passing assertions and counting!

But phantomjs is no real browser so it should not be the only answer to “Is my module working in browsers?”. To solve this, we have an npm run dev task that will expose our tests in a simple web server accessible by any browser:

All of theses features are provided by defunctzombie/zuul
All of theses features are provided by defunctzombie/zuul

Finally, if you have virtual machines, you can test in any browser you want, all locally:

Here's a VirtualBox setup created with xdissent/ievms
Here’s a VirtualBox setup created with xdissent/ievms

What comes next after setting up a good local development workflow? Continuous integration setup!

Continuous integration

defunctzombie/zuul supports running tests using Saucelabs browsers. Saucelabs provides browsers as a service (manual testing or Selenium automation). It also has a nice OSS plan called Opensauce. We patched our .zuul.yml configuration file to specify what browsers we want to test. You can find all the details in zuul’s wiki.

Now there’s only one missing piece: Travis CI. Travis runs our tests in all browsers defined in our .zuul.yml file. Our travis.yml looks like this:

All platforms are tested using travis matrixes
All platforms are tested using a travis build matrix

Right now tests are taking a bit too long so we will soon split them between desktop and mobile.

We also want to to tests on pull requests using only latest stable versions of all browsers. So that it does not takes too long. As a reward, we get a nice badge to display in our Github readme:

Gray color means the test is currently running
Gray color means the test is currently running

Challenge #2: redesign and rewrite

Once we had a usable testing stack, we started our rewrite, the V3 milestone on Github.


We dropped the new AlgoliaSearch() usage in favor of just algoliasearch(). It allows us to hide implementation details to our API users.


new AlgoliaSearch(applicationID, apiKey, opts);


algoliasearch(applicationID, apiKey, opts);

Callback convention

Our JavaScript client now follows the error-first and callback-last conventions. We had to break some methods to do so.


client.method(param, callback, param, param);


client.method(params, param, param, params, callback);

This allows our callback lovers to use libraries like caolan/async very easily.

Promises and callbacks support

Promises are a great way to handle the asynchronous flow of your application.

We implemented both promises and callbacks, it was nearly a no-brainer. In every command, if you do not provide a callback, you get a Promise. We use native promises in compatible environments and jakearchibald/es6-promise as a polyfill.

AlgoliaSearchHelper removal

The main library was also previously exporting window.AlgoliaSearchHelper to ease the development of awesome search UIs. We externalized this project and it now has now has a new home at algolia/algoliasearch-helper-js.


UMD: JavaScript modules that run anywhere

The previous version was directly exporting multiple properties in the window object. As we wanted our new library to be easily compatible with a broad range of module loaders, we made it UMD compatible. It means our library can be used:

This was achieved by writing our code in a CommonJS style and then use the standalone build feature of browserify.

see browserify usage
see browserify usage

Multiple builds

Our JavaScript client isn’t only one build, we have multiple builds:

Previously this was all handled in the main JavaScript file, leading to unsafe code like this:


How do we solve this? Using inheritance! JavaScript prototypal inheritance is the new code smell in 2015. For us it was a good way to share most of the code between our builds. As a result every entry point of our builds are inheriting from the src/AlgoliaSearch.js.

Every build then need to define how to:

Using a simple inheritance pattern we were able to solve a great challenge.

Example of the vanilla JavaScript build
Example of the vanilla JavaScript build

Finally, we have a build script that will generate all the needed files for each environment.

Challenge #3: backward compatibility

We could not completely modernize our JavaScript clients while keeping a full backward compatibility between versions. We had to break some of the previous usages to level up our JavaScript stack.

But we also wanted to provide a good experience for our previous users when they wanted to upgrade:

  • we re-exported previous constructors like window.AlgoliaSearch*. But we now throw if it’s used
  • we wrote a clear migration guide for our existing Node.js and JavaScript users
  • we used npm deprecate on our previous Node.js module to inform our current user base that we moved to a new client
  • we created legacy branches so that we can continue to push critical updates to previous versions when needed

Make it isomorphic!

Our final step was to make our JavaScript client work in both Node.js and the browser.

Having separated the builds implementation helped us a lot, because the Node.js build is a regular build only using the http module from Node.js.

Then we only had to tell module loaders to load index.js on the server and src/browser/.. in browsers.

This last step was done by configuring browserify in our package.json:

the browser field from browserify also works in webpack
the browser field from browserify also works in webpack

If you are using the algoliasearch module with browserify or webpack, you will get our browser implementation automatically.

The faux-jax library is released under MIT like all our open source projects. Any feedback or improvement idea are welcome, we are dedicated to make our JS client your best friend 🙂

About the authorVincent Voyer

Vincent Voyer

Engineering Manager

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