How to mock the imports of an ES6 module?

Question

I have the following ES6 modules:

network.js

export function getDataFromServer() {
  return ...
}

widget.js

import { getDataFromServer } from 'network.js';

export class Widget() {
  constructor() {
    getDataFromServer("dataForWidget")
    .then(data => this.render(data));
  }

  render() {
    ...
  }
}

I'm looking for a way to test Widget with a mock instance of getDataFromServer. If I used separate <script>s instead of ES6 modules, like in Karma, I could write my test like:

describe("widget", function() {
  it("should do stuff", function() {
    let getDataFromServer = spyOn(window, "getDataFromServer").andReturn("mockData")
    let widget = new Widget();
    expect(getDataFromServer).toHaveBeenCalledWith("dataForWidget");
    expect(otherStuff).toHaveHappened();
  });
});

However, if I'm testing ES6 modules individually outside of a browser (like with Mocha + babel), I would write something like:

import { Widget } from 'widget.js';

describe("widget", function() {
  it("should do stuff", function() {
    let getDataFromServer = spyOn(?????) // How to mock?
    .andReturn("mockData")
    let widget = new Widget();
    expect(getDataFromServer).toHaveBeenCalledWith("dataForWidget");
    expect(otherStuff).toHaveHappened();
  });
});

Okay, but now getDataFromServer is not available in window (well, there's no window at all), and I don't know a way to inject stuff directly into widget.js's own scope.

So where do I go from here?

1. Is there a way to access the scope of widget.js, or at least replace its imports with my own code?
2. If not, how can I make Widget testable?

---

Stuff I considered:

a. Manual dependency injection.

Remove all imports from widget.js and expect the caller to provide the deps.

export class Widget() {
  constructor(deps) {
    deps.getDataFromServer("dataForWidget")
    .then(data => this.render(data));
  }
}

I'm very uncomfortable with messing up Widget's public interface like this and exposing implementation details. No go.

---

b. Expose the imports to allow mocking them.

Something like:

import { getDataFromServer } from 'network.js';

export let deps = {
  getDataFromServer
};

export class Widget() {
  constructor() {
    deps.getDataFromServer("dataForWidget")
    .then(data => this.render(data));
  }
}

then:

import { Widget, deps } from 'widget.js';

describe("widget", function() {
  it("should do stuff", function() {
    let getDataFromServer = spyOn(deps.getDataFromServer)  // !
      .andReturn("mockData");
    let widget = new Widget();
    expect(getDataFromServer).toHaveBeenCalledWith("dataForWidget");
    expect(otherStuff).toHaveHappened();
  });
});

This is less invasive but requires me to write a lot of boilerplate for each module, and there's still a risk of me using getDataFromServer instead of deps.getDataFromServer all the time. I'm uneasy about it, but that's my best idea so far.

Answer 1

@carpeliam is correct but note that if you want to spy on a function in a module and use another function in that module calling that function, you need to call that function as part of the exports namespace otherwise the spy won't be used.

Wrong example:

// mymodule.js

export function myfunc2() {return 2;}
export function myfunc1() {return myfunc2();}

// tests.js
import * as mymodule

describe('tests', () => {
    beforeEach(() => {
        spyOn(mymodule, 'myfunc2').and.returnValue = 3;
    });

    it('calls myfunc2', () => {
        let out = mymodule.myfunc1();
        // out will still be 2
    });
});

Right example:

export function myfunc2() {return 2;}
export function myfunc1() {return exports.myfunc2();}

// tests.js
import * as mymodule

describe('tests', () => {
    beforeEach(() => {
        spyOn(mymodule, 'myfunc2').and.returnValue = 3;
    });

    it('calls myfunc2', () => {
        let out = mymodule.myfunc1();
        // out will be 3 which is what you expect
    });
});

Answer 2

@vdloo's answer got me headed in the right direction, but using both commonjs "exports" and ES6 module "export" keywords together in the same file did not work for me (webpack v2 complains). Instead, I'm using a default (named variable) export wrapping all of the individual named module exports and then importing the default export in my tests file. I'm using the following export setup with mocha/sinon and stubbing works fine without needing rewire, etc.:

// MyModule.js
let MyModule;

export function myfunc2() { return 2; }
export function myfunc1() { return MyModule.myfunc2(); }

export default MyModule = {
  myfunc1,
  myfunc2
}

// tests.js
import MyModule from './MyModule'

describe('MyModule', () => {
  const sandbox = sinon.sandbox.create();
  beforeEach(() => {
    sandbox.stub(MyModule, 'myfunc2').returns(4);
  });
  afterEach(() => {
    sandbox.restore();
  });
  it('myfunc1 is a proxy for myfunc2', () => {
    expect(MyModule.myfunc1()).to.eql(4);
  });
});

Answer 3

I implemented a library that attempts to solve the issue of run-time mocking of Typescript class imports without needing the original class to know about any explicit dependency injection.

The library uses the import * as syntax and then replaces the original exported object with a stub class. It retains type safety so your tests will break at compile time if a method name has been updated without updating the corresponding test.

This library can be found here: ts-mock-imports.

Answer 4

I've started employing the import * as obj style within my tests, which imports all exports from a module as properties of an object which can then be mocked. I find this to be a lot cleaner than using something like rewire or proxyquire or any similar technique. I've done this most often when needing to mock Redux actions, for example. Here's what I might use for your example above:

import * as network from 'network.js';

describe("widget", function() {
  it("should do stuff", function() {
    let getDataFromServer = spyOn(network, "getDataFromServer").andReturn("mockData")
    let widget = new Widget();
    expect(getDataFromServer).toHaveBeenCalledWith("dataForWidget");
    expect(otherStuff).toHaveHappened();
  });
});

If your function happens to be a default export, then import * as network from './network' would produce {default: getDataFromServer} and you can mock network.default.

Answer 5

I have found this syntax to be working:

My module:

// mymod.js
import shortid from 'shortid';

const myfunc = () => shortid();
export default myfunc;

My module's test code:

// mymod.test.js
import myfunc from './mymod';
import shortid from 'shortid';

jest.mock('shortid');

describe('mocks shortid', () => {
  it('works', () => {
    shortid.mockImplementation(() => 1);
    expect(myfunc()).toEqual(1);
  });
});

See the doc.