How to unit test a Node.js module that requires other modules and how to mock the global require function?


Question

This is a trivial example that illustrates the crux of my problem:

var innerLib = require('./path/to/innerLib');

function underTest() {
    return innerLib.doComplexStuff();
}

module.exports = underTest;

I am trying to write a unit test for this code. How can I mock out the requirement for the innerLib without mocking out the require function entirely?

So this is me trying to mock out the global require and finding out that it won’t work even to do that:

var path = require('path'),
    vm = require('vm'),
    fs = require('fs'),
    indexPath = path.join(__dirname, './underTest');

var globalRequire = require;

require = function(name) {
    console.log('require: ' + name);
    switch(name) {
        case 'connect':
        case indexPath:
            return globalRequire(name);
            break;
    }
};

The problem is that the require function inside the underTest.js file has actually not been mocked out. It still points to the global require function. So it seems that I can only mock out the require function within the same file I’m doing the mocking in. If I use the global require to include anything, even after I’ve overridden the local copy, the files being required will still have the global require reference.

1
147
8/18/2019 2:23:28 AM

Accepted Answer

You can now!

I published proxyquire which will take care of overriding the global require inside your module while you are testing it.

This means you need no changes to your code in order to inject mocks for required modules.

Proxyquire has a very simple api which allows resolving the module you are trying to test and pass along mocks/stubs for its required modules in one simple step.

@Raynos is right that traditionally you had to resort to not very ideal solutions in order to achieve that or do bottom-up development instead

Which is the main reason why I created proxyquire - to allow top-down test driven development without any hassle.

Have a look at the documentation and the examples in order to gauge if it will fit your needs.

169
8/2/2012 11:56:32 AM

A better option in this case is to mock methods of the module that gets returned.

For better or worse, most node.js modules are singletons; two pieces of code that require() the same module get the same reference to that module.

You can leverage this and use something like sinon to mock out items that are required. mocha test follows:

// in your testfile
var innerLib  = require('./path/to/innerLib');
var underTest = require('./path/to/underTest');
var sinon     = require('sinon');

describe("underTest", function() {
  it("does something", function() {
    sinon.stub(innerLib, 'toCrazyCrap').callsFake(function() {
      // whatever you would like innerLib.toCrazyCrap to do under test
    });

    underTest();

    sinon.assert.calledOnce(innerLib.toCrazyCrap); // sinon assertion

    innerLib.toCrazyCrap.restore(); // restore original functionality
  });
});

Sinon has good integration with chai for making assertions, and I wrote a module to integrate sinon with mocha to allow for easier spy/stub cleanup (to avoid test pollution.)

Note that underTest cannot be mocked in the same way, as underTest returns only a function.

Another option is to use Jest mocks. Follow up on their page


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