Stubbing module dependencies in ES2015 unit tests using Sinon.js or TestDouble.js.

README.md

I was all set up for the pain of stubbing Node.js dependencies, but you can just use the import * as blah from 'blah' syntax to obtain an object with properties that can be stubbed using either TestDouble or Sinon.

The object is shared between modules at runtime, so modifications will propagate everywhere. Be sure the use Sinon's restore() or TestDouble's reset() methods to return the module to it's normal state so that it doesn't bleed into other tests.

If you need to stub a default module function, then use this import

import * as kphToMphModule from '../src/kphToMph';

and stub with testdouble like this:

const kphToMph = td.replace(kphToMphModule, 'default');

or with Sinon:

const kphToMph = sinon.stub(kphToMphModule, 'default');

Car.js

// in /src directory

import {kphToMph} from './convert';

class Car {
  constructor(speed) {
    this.speed = speed;
  }

  getSpeedInMph() {
    return kphToMph(this.speed);
  }
}

export default Car;

convert.js

// In /src directory

export function kphToMph(speed) {
  // Real function here
  return 0;
}

package.json

{
  "devDependencies": {
    "babel-cli": "^6.14.0",
    "babel-preset-es2015": "^6.14.0",
    "babel-register": "^6.14.0",
    "mocha": "^3.0.2",
    "sinon": "^1.17.5",
    "testdouble": "^1.6.1"
  },
  "scripts": {
    "test": "mocha --compilers js:babel-register"
  }
}

test-sinon.js

// Goes in /test directory

import assert from 'assert';
import sinon from 'sinon';

// Subject
import Car from '../src/Car';

// Dependency of subject - import whole module as an object
import * as convert from '../src/convert';

describe('Car', () => {
  afterEach(() => {
    // Need to reset any replaced methods
    convert.kphToMph.restore();
  });

  it('returns speed converted to MPH', () => {
    // Replace the internal dependency of Car
    const getSpeedInMph = sinon.stub(convert, 'kphToMph');
    // When the dependent method is called, return a fixed value
    getSpeedInMph.withArgs(50).returns(31);

    const car = new Car(50);

    // Call the function that uses the dependency
    // This will now call the replaced function
    const actual = car.getSpeedInMph();

    // Our expected value
    const expected = 31;

    assert.equal(actual, expected);
  });
});

test-testdouble.js

// Goes in /test directory

import assert from 'assert';
import td from 'testdouble';

// Subject
import Car from '../src/Car';

// Dependency of subject - import whole module as an object
import * as convert from '../src/convert';

describe('Car', () => {
  afterEach(() => {
    // Need to reset any replaced methods
    td.reset();
  });

  it('returns speed converted to MPH', () => {
    // Replace the internal dependency of Car
    const kphToMph = td.replace(convert, 'kphToMph');
    // When the dependent method is called, return a fixed value
    td.when(kphToMph(50)).thenReturn(31);

    const car = new Car(50);

    // Call the function that uses the dependency
    // This will now call the replaced function
    const actual = car.getSpeedInMph();

    // Our expected value
    const expected = 31;

    assert.equal(actual, expected);
  });
});

Does not work with add-module-exports preset, found out a hard way :(

https://ide.c9.io/strajk/node-stubbing-module-deps

looks pretty cool. unsure i'll be able to do any better with testdouble.js since import is (theoretically) totally static, even if there might be a sneaky way to accomplish this as long as folks are transpiling it in