Mocha tests and require self

Feb 11, 2016  

In my last post I showed how to make require-self work smoothly with Wallaby.js.

But if you are like me, you also want to continue to run your mocha tests on a regular basis, for instance on a CI server. In this case, some additional work is required to get require-self to work smoothly with mocha too.

My test script is defined like this and is started with npm test:

  "script": {
    "test": "mocha src/test/**/*.js"

I have a test/mocha.opts file which contains the basic settings for running mocha with Babel. Add a --require option to inject test-require-patch.js:

--compilers js:babel-core/register
--require ./test/test-require-patch.js

And here is the test/test-require-patch.js source file:

/*globals __dirname */
'use strict';

var path = require ('path');
var root = path.join (__dirname, '..');
var packageConfig = require (path.join (root, 'package.json'));
var packageName =;
var modulePrototype = require ('module').Module.prototype;

if (!modulePrototype._originalRequire) {
  modulePrototype._originalRequire = modulePrototype.require;
  modulePrototype.require = function (filePath) {
    if (filePath === packageName) {
      filePath = path.join (root, 'src');
    return (this, filePath);

It will redirect any reference to the module using require('foo') to src/index.js.


I shared an interesting discussion with yortus, require-self’s author, in order to see if there is a better solution than patching require(). It appears that the easiest solution is to run the tests from the transpiled output rather than from the sources. It makes sense, since we want to test what npm will publish. And, moreover, it’s easy to configure:

  "script": {
    "compile": "babel -d lib/ src/",
    "prepublish": "require-self && npm run compile",
    "test": "npm run compile && mocha --harmony lib/test/**/*.js"

Notice how we go through the compile step first, then start mocha on the transpiled output. The --harmony flag is needed when this is run on Node v4.x which does not by default understand ES2015 features. With this setup, no patching of require is required.