binance-api/__tests__/main.test.ts

import { Delays, greeter } from '../src/main.js'

describe('greeter function', () => {
  const name = 'John'
  let hello: string

  let timeoutSpy: jest.SpyInstance

  // Act before assertions
  beforeAll(async () => {
    // Read more about fake timers
    // http://facebook.github.io/jest/docs/en/timer-mocks.html#content
    // Jest 27 now uses "modern" implementation of fake timers
    // https://jestjs.io/blog/2021/05/25/jest-27#flipping-defaults
    // https://github.com/facebook/jest/pull/5171
    jest.useFakeTimers()
    timeoutSpy = jest.spyOn(global, 'setTimeout')

    const p: Promise<string> = greeter(name)
    jest.runOnlyPendingTimers()
    hello = await p
  })

  // Teardown (cleanup) after assertions
  afterAll(() => {
    timeoutSpy.mockRestore()
  })

  // Assert if setTimeout was called properly
  it('delays the greeting by 2 seconds', () => {
    expect(setTimeout).toHaveBeenCalledTimes(1)
    expect(setTimeout).toHaveBeenLastCalledWith(
      expect.any(Function),
      Delays.Long,
    )
  })

  // Assert greeter result
  it('greets a user with `Hello, {name}` message', () => {
    expect(hello).toBe(`Hello, ${name}`)
  })
})