Require a modules implementation file when writing tests
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Here at newscorpau we use a simple approach to dependency injection when writing modules for our node.js based applications.

Internally it has become known as the _implementation approach and was first introduced by @drew-walker at a time when mocking required dependencies became convoluted.

The intent with this module is to formalise the approach and provide an easy way of writing tests for modules which follow this pattern.

dependency injection

There has been numerous attempts to solve this. However these often involve manipulating the require.cache and adding un-necessary complexity.

With the _implementation approach dependency injection becomes far simpler. That's if you don't mind splitting a modules dependencies and it's actual implementation between two files.

I know what your thinking, "But hey that adds an extra file I shouldn't really need" and you would be right and I thought the same initially however the benefits far outway the need for that extra file.

It would be awesome if node itself had a supported DI approach baked in however as with most wishes these can often be solved by userland and this is just another of those userland attempts :-)

So what does this mean for your code? Not much changes really.


var path       = require('path'),
    currentEnv = process.env.NODE_ENV;

module.exports = function (env) {
    env = env || currentEnv;
    var config = path.join(__dirname, env);
    return require(env);



var path       = require('path'),
    currentEnv = process.env.NODE_ENV;

module.exports = require('./module_implementation')
                    (require, __dirname, path, currentEnv);


module.exports = function (require, path, dirname, currentEnv) {

    var self = function (env)
            env = env || currentEnv;
            var config = path.join(dirname, env);
            return require(env);

    return self;




It now becomes fairly straightforward to require a modules implementation and begin to mock it's dependencies.

var test           = require('tape'),
    implementation = require('implementation')('lib/config');

test('path.join called correctly when no environment specified', function (t) {

    implementation(function () {
    }, {
        join : function(dirname, env) {
            t.equal(dirname, 'some/dir', 'should match __dirname');
            t.equal(env, 'development', 'should match default environment');
    }, 'some/dir', 'development');


For a real example checkout how this module was written. It follows the _implementation approach.


In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using npm test

Release History

  • 0.1.0 Initial release


Copyright (c) 2015 News Corp Australia. Licensed under the MIT license.