Stub function test with sinon/nodejs

Question

I have a problem with sketching a function with sinon.

The problem is to return a result or to throw an error in a function of another function.

As below:

service.js

async function functionA() {
  var resultB = functionB();
}

function functionB() {
  return "FuncB";
}

module.exports = {
  functionA,
  functionB
}

service.test.js

const { assert } = require("chai");
const sinon = require("sinon");
const service = require("./service");

it('Should return error.', async function() {
  var stub = sinon.stub(service, 'functionB').returns('functionC');
  var functionTotest = service.functionA();

  assert(stub.calledOn(functionTotest));
});

The function is not simulating an error or the return that I set.

The stub does not work and enters the function.

Answer 1

Here is the unit test solution:

service.js:

async function functionA() {
  var resultB = exports.functionB();
  return resultB;
}

function functionB() {
  return 'FuncB';
}

exports.functionA = functionA;
exports.functionB = functionB;

service.test.js:

const { assert, expect } = require('chai');
const sinon = require('sinon');
const service = require('./service');

it('Should return error.', async function() {
  var stub = sinon.stub(service, 'functionB').returns('functionC');
  var actual = await service.functionA();
  expect(actual).to.be.equal('functionC');
  expect(stub.calledOnce).to.be.true;
});

Unit test result with coverage report:

  ✓ Should return error.

  1 passing (9ms)

-----------------|----------|----------|----------|----------|-------------------|
File             |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
-----------------|----------|----------|----------|----------|-------------------|
All files        |    92.31 |      100 |    66.67 |    92.31 |                   |
 service.js      |       80 |      100 |       50 |       80 |                 7 |
 service.test.js |      100 |      100 |      100 |      100 |                   |
-----------------|----------|----------|----------|----------|-------------------|

Source code: https://github.com/mrdulin/mocha-chai-sinon-codelab/tree/master/src/stackoverflow/56759906