Mocking with Jest: Taking Advantage of the Module System

[utterances-bot]

Mocking with Jest: Taking Advantage of the Module System

Using Jest to reimplement certain modules when you don't want their usual behavior during testing.

https://silvenon.com/blog/mocking-with-jest/modules/

[zhao-li]

Thank you again for writing this up. This and the first article is a life-saver for someone who got mocks to work but never quite understood when to use which method of mocking.

I went through your article in great detail and wanted to share some issues I ran in to, in case it can help the next person reading your article:

• the getSillyWalk method is a bit harder to grok than just returning a Math.random()

  getSillyWalk(numberOfSteps) {
  const steps = []
  for (let i = 0; i < numberOfSteps; i++) {
    if (steps[steps.length - 1] !== 'left') {
      steps.push('left')
    } else {
      steps.push('right')
    }
  }
  // shuffle
  return steps.sort(() => 0.5 - Math.random())

• it was an extra step for me to figure out, but maybe it was left out to save space. In case it helps others:

  it('gets the real meaning of life', () => {
    jest.dontMock('./monty-python')
    const RealMontyPython = require('./monty-python')
    // ... [filling out this part that was omitted for brevity; not sure if it's what was intended or not]
    const mathRandomSpy = jest.spyOn(Math, 'random')
    mathRandomSpy.mockImplementation(() => '¯\_(ツ)_/¯')
    const montyPython = new RealMontyPython()
    expect(montyPython.getTheMeaningOfLife()).toBe('¯\_(ツ)_/¯')
    mathRandomSpy.mockRestore()
    jest.resetModules()
  })

• the section on manual mocks required me to fill in some blanks on how the test actually looked like. It was do-able, but not sure if you want to just lay it out for the next newbie that comes along.

Also, your explanation of how mocks are hoisted was super helpful. I ran into that error and conflated it with a bunch of other things. Your explanation clarified everything up.

Thank you again for writing all of this out so simply and succinctly. 🙏

[silvenon]

Thank you so much for you generous feedback! I updated my posts based on what you said 🚀 hopefully I covered everything that you had trouble with, and I'm so happy my posts helped you understand these features!