How to mock npm module with sinon/mocha

Question

I'm trying to test a function that calls the module cors. I want to test that cors would be called. For that, I'd have to stub/mock it.

Here is the function cors.js

const cors = require("cors");

const setCors = () => cors({origin: 'http//localhost:3000'});
module.exports = { setCors }

My idea of testing such function would be something like

cors.test.js

  describe("setCors", () => {
    it("should call cors", () => {
      sinon.stub(cors)

      setCors();
      expect(cors).to.have.been.calledOnce;

    });
  });

Any idea how to stub npm module?

Answer 1

You can use mock-require or proxyquire

Example with mock-require

const mock = require('mock-require')
const sinon = require('sinon')

describe("setCors", () => {
  it("should call cors", () => {
    const corsSpy = sinon.spy();
    mock('cors', corsSpy);

    // Here you might want to reRequire setCors since the dependancy cors is cached by require
    // setCors = mock.reRequire('./setCors');

    setCors();
    expect(corsSpy).to.have.been.calledOnce;
    // corsSpy.callCount should be 1 here

    // Remove the mock
    mock.stop('cors');
  });
});

If you want you can define the mock on top of describe and reset the spy using corsSpy.reset() between each tests rather than mocking and stopping the mock for each tests.