Javascript Mocha/Sinon fake XMLHttpRequest to return object in middle of tested method

Question

Hello so i am new to using mocha/sinon and i have this issue

i am testing this method

export async function onConfigEvent(event: configEvent, otherConfig: otherConfig) {
  var myConfig = event.config as Config;

  const info = await fetchInfo(otherConfig.detailsPath, myConfig.Id);

  if (!isDefined(info)) {
    console.log('info is undefined.');
    return;
  }
}

this method is actually quite much bigger with plenty of if statements so i want to test every path of it. the problem i have is with the fetchInfo() call. this function will call a function that creates an url and so on and then calls another function the makes the actual call with a promise. what i need to to is to fake this fetchInfo and make it return what i want for this instance i want it to return undefined so it goes into the if and in other cases i want it to return a faked model like if it succeeded. but i have no idea on how to do it and i have searched for a long while and started trying with sinon.stub but how would i implement that here?

this is how my test looks so far

it('should log info undefined', () => {

let event: configEvent = {
  type: events.Config,
  config: { ["Id"]: 361 }
}

let fetch = sinon.stub(fetchResource);

let otherConfig = getOtherConfig();

let spy = sinon.spy(console, 'log');

onConfigEvent(event, otherConfig)

assert(spy.calledWith('info is undefined.'));

spy.restore();

});

where fetchResource is the function that fetchInfo calls but this does nothing

Answer 1

You need to stub fetchInfo function. sinon.js doesn't support stub a standalone function directly. Need to use rewire package.

E.g.

index.ts:

function isDefined(obj) {
  return typeof obj !== 'undefined';
}

export async function fetchInfo(url, id) {
  //
}

export async function onConfigEvent(event, otherConfig) {
  const myConfig = event.config;

  const info = await fetchInfo(otherConfig.detailsPath, myConfig.Id);

  if (!isDefined(info)) {
    console.log('info is undefined.');
    return;
  }
}

index.test.ts:

import rewire from 'rewire';
import sinon from 'sinon';

describe('60828989', () => {
  afterEach(() => {
    sinon.restore();
  });
  it('should print log if info is undefined', async () => {
    const mod = rewire('./');
    const fetchInfoStub = sinon.stub().resolves();
    const logSpy = sinon.spy(console, 'log');
    mod.__set__('fetchInfo', fetchInfoStub);
    const event = { config: { id: 1 } };
    const otherConfig = { detailsPath: 'http:/localhost' };
    await mod.onConfigEvent(event, otherConfig);
    sinon.assert.calledWithExactly(logSpy, 'info is undefined.');
  });
  it('should do nothing if info is defined', async () => {
    const mod = rewire('./');
    const fetchInfoStub = sinon.stub().resolves({});
    const logSpy = sinon.spy(console, 'log');
    mod.__set__('fetchInfo', fetchInfoStub);
    const event = { config: { id: 1 } };
    const otherConfig = { detailsPath: 'http:/localhost' };
    await mod.onConfigEvent(event, otherConfig);
    sinon.assert.notCalled(logSpy);
  });
});

unit test results with coverage report:

  60828989
info is undefined.
    ✓ should print log if info is undefined (124ms)
    ✓ should do nothing if info is defined


  2 passing (150ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |   66.67 |     100 |                   
 index.ts |     100 |      100 |   66.67 |     100 |                   
----------|---------|----------|---------|---------|-------------------

source code: https://github.com/mrdulin/expressjs-research/tree/master/src/stackoverflow/60828989