BigFrontEnd Category 8 Jest Implementation Questions

Introduction

This blog post summarizes the Jest implementation related questions found on BigFrontEnd.Dev.

1.implement jest.spyOn()
2.toBe() or not.toBe()

1.implement `jest.spyOn()`

38.https://bigfrontend.dev/problem/implement-spyOn If you did unit test before, you must be familiar with Spy.

You are asked to create a spyOn(object, methodName), which works the same as jest.spyOn().

To make it simple, here are the 2 requirements of spyOn

original method should be called when spied one is called spy should have a calls array, which holds all the arguments in each call. Code to explain everything.

const obj = {
  data: 1,
  increment(num) {
    this.data += num
  },
}
const spy = spyOn(obj, 'increment')
obj.increment(1)
console.log(obj.data) // 2
obj.increment(2)
console.log(obj.data) // 4
console.log(spy.calls)
// [ [1], [2] ]

Solution:

/**
 * @param {object} obj
 * @param {string} methodName
 */
function spyOn(obj, methodName) {
  const calls = []
  const originalMethod = obj[methodName]
  if (!originalMethod || typeof originalMethod !== 'function') {
    throw new Error(`${methodName} does not exist`)
  }
  obj[methodName] = (...args) => {
    calls.push(args)
    originalMethod(...args)
  }
  return { calls }
}

2.`toBe()` or `not.toBe()`

161.https://bigfrontend.dev/problem/jest-assertion Here are some simple Jest test code.

expect(3).toBe(3) // ✅
expect(4).toBe(3) // ❌

We can reverse it with not.

expect(3).not.toBe(3) // ❌
expect(4).not.toBe(3) // ✅

Please implement myExpect() to support toBe() and also not.

Solution:

/**
 * interface Matcher {
 *  toBe(data: any): void
 * }
 */
/**
 * @param {any} input
 * @returns {Matcher & {not: Matcher}}
 */
function myExpect(input) {
  // x === input will fail with 0, -0 (return true) and NaN, NaN (return false)
  const isEqual = (x, input) => {
    return Object.is(x, input)
  }
  return {
    toBe: (x) => {
      if (!isEqual(x, input)) {
        throw new Error()
      }
    },
    not: {
      toBe: (x) => {
        if (isEqual(x, input)) {
          throw new Error()
        }
      },
    },
  }
}

Introduction

1.implement jest.spyOn()

2.toBe() or not.toBe()

1.implement `jest.spyOn()`

2.`toBe()` or `not.toBe()`