如何模拟模块导出功能

Question

I have a function that is exported like this:

// myFunc.js
....
....
module.exports = myFunc;

And then in another file I have:

// main.js
const myFunc = require('../../myFunc');
...
...
myFunc.then( .... )

How can I mock myFunc in myFunc.js with another function? I've tried:

sinon.stub(myFuncFilePath).callsFake(myOtherFuncFilePath);

But it's not working, apparently because of the way myFunc is exported. I can't change the way it is exported in myFunc.js , so how else can I mock it?

Answer 1

sinon doesn't support stub a function directly. You need use proxyquire to rewire the module:

Eg myFunc.js :

async function myFunc() {
  console.log('myFunc');
}

module.exports = myFunc;

main.js :

const myFunc = require('./myFunc');

function main() {
  return myFunc().then(res => console.log(res));
}

module.exports = main;

main.spec.js :

const sinon = require('sinon');
const { expect } = require('chai');
const proxyquire = require('proxyquire');

describe('main', () => {
  it('should stub myFunc', async () => {
    const myFuncStub = sinon.stub().resolves('fake data');
    const main = proxyquire('./main', {
      './myFunc.js': myFuncStub
    });
    const logSpy = sinon.spy(console, 'log');
    const actual = await main();
    expect(actual).to.be.undefined;
    expect(myFuncStub.calledOnce).to.be.true;
    expect(logSpy.calledWith('fake data')).to.be.true;
  });
});

Unit test result with coverage report:

  main
fake data
    ✓ should stub myFunc (44ms)


  1 passing (49ms)

--------------|----------|----------|----------|----------|-------------------|
File          |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
--------------|----------|----------|----------|----------|-------------------|
All files     |    94.44 |      100 |       80 |    94.12 |                   |
 main.js      |      100 |      100 |      100 |      100 |                   |
 main.spec.js |      100 |      100 |      100 |      100 |                   |
 myFunc.js    |       50 |      100 |        0 |       50 |                 2 |
--------------|----------|----------|----------|----------|-------------------|

Source code: https://github.com/mrdulin/mocha-chai-sinon-codelab/tree/master/src/stackoverflow/57013728