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Test your Leaflet applications with Mocha

Pretty much like n1k0, I feel like I had learned Javascript three or four times, from the alert() back in 1997 to this article about automatic testing. I must admit that, lately, most of my progress in Javascript comes from using and hacking Leaflet [1], but I hadn't gone as far as unit testing until now !

These are some notes about me getting started with using Mocha and Leaflet. If what you read is not clear or simply wrong, please let me know or fork it directly so that everybody can learn !


  • Test your Javascript code to prevent regressions, just as you already do with Python ;
  • Run test suites from command-line, especially for CI ;
  • Learn something new and practical !

There are many ways to achieve this, you might have spotted QUnit or Jasmine. We also like CasperJS, coupled with resurectio, but this would be more adapted to navigation automation or functional tests.

I chose Mocha since it seems to be well suited for unit tests and command-line usage. And since there is a pull-request for switching from Jasmine to Mocha in Leaflet core... why not !

First, run the suite

Get your hand on the npm command (comes with nodejs package in Ubuntu)

Create a package.json file with your application description. There are plenty of examples, just make sure you require the right stuff :

  "name": "yourapp",
  "version": "0.0.1",
  "description": "your app",
  "main": "yourapp.js",
  "scripts": {
    "test": "make test",
  "dependencies": {
    "leaflet": "*"
  "devDependencies": {
    "mocha": "*"

And fetch !

npm install

Create yourapp.js with simple and stupid stuff :

L.YourApp = {
    compute: function () { return 2; }

And create a test for it in test/beginner.js :

// Use require only if available (ran from Node)
if (typeof require == 'function') {
    var assert = require('assert'),
    L = require('leaflet/src/Leaflet');
    L.YourApp = require('./../yourapp').YourApp;

// Test function call
describe('compute', function() {
  it('should be ok', function(done) {
     assert.equal(2, L.YourApp.compute());

Admire the result !


Make it run in the browser too

So far we do not rely too much on Leaflet :) But in a real application test, we will quickly need a L.Map instance, along with a DOM most probably.

By turning on the Mocha HTML runner, we can indeed run tests from a web browser. But since the console remains one of our goals, we add mocha-phantomjs in the scene !

Install phantomjs and add it to the PATH (the Ubuntu package does that for you). Then modify your package.json to add mocha-phantomjs as a devDependency. Re-run npm install to fetch it.

With mocha-phantomjs, we will be able to run tests from within a browser and from the command-line. The entry point will be the following test/index.html:

<!DOCTYPE html>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="../node_modules/mocha/mocha.css" />
    <div id="mocha"></div>
    <div id="map" style="display: none; height: 300px"></div>
    <script src="../node_modules/mocha/mocha.js"></script>
    <script src="../node_modules/leaflet/debug/leaflet-include.js"></script>
    <script src="../yourapp.js"></script>

        var map ='map').fitWorld();

    <script src="begginner.js"></script>
      (window.mochaPhantomJS || window.mocha).run();

Open the page locally or run in console with :

@./node_modules/mocha-phantomjs/bin/mocha-phantomjs test/index.html

PhantomJS is installed by default on Travis by the way :)

Spying and mocking

One of the popular tools in JS testing is Sinon.js. There are many useful features allowing to spy and mock behaviour of your application components or dependencies (events, AJAX requests, errors, timers, etc.)

For example, let's test that events are thrown as we expect :

L.YourApp.snap = function (marker) {'snap');

Test event with a spy callback :

describe('snap', function() {
  it('event is thrown', function(done) {
     var marker = L.marker([0, 0]),
         callback = sinon.spy();
     marker.on('snap', callback);



Faking user inputs is also possible using happen :

describe('zoom', function() {
  it('zooms-in with double click', function(done) {
     assert.equal(0, map.getZoom());

     map.on('zoomend', function () {
        assert.equal(1, map.getZoom());'zoomend');

     // Simulate double-click

Real world example

Benjamin Becquet implemented some linear referencing utilities for Leaflet. So did we last year at Makina Corpus ! We thus decided to merge our code base in a proper way :)

We both are making our first steps with Mocha, and didn't really started to build up the whole code, but you still can have a look at the repository, for its Makefile, Travis setup, usage of JSDocs or Chai.js...

[1]By the way, Secrets of the Javascript Ninja by John Resig and Bear Bibeault is a wonderful book !

Comments !