AsyncSerializer.js

class AsyncSerializer {
  constructor() {
    this._previousPromise = Promise.resolve(null)
  }

  serialize = f => {
    const newPromise = this._previousPromise.then(() => f())
    this._previousPromise = newPromise
    return newPromise
  }
}

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))

it('no matt no', async () => {
  const asyncSerializer = new AsyncSerializer()

  let a = 0
  asyncSerializer.serialize(async () => {
    const aCopy = a
    await sleep(1000)
    a = aCopy + 1
  })
  const resultPromise = asyncSerializer.serialize(async () => {
    const aCopy = a
    await sleep(50)
    a = aCopy + 1
  })
  await resultPromise
  expect(resultPromise).toBe(2)
})