TypeScript 回调类型

Question

I have a case like this:

function createCar(name: string, callback: () => void)

function buildEngine(name: string): Engine

function createCarWithEngine(carName: string, engineName: string, callback: (param: Engine) => void) {
  let createdEngine = createdEngines.find((engine) => engine.name === engineName)
  if (!createdEngine) createdEngine = buildEngine(engineName)

  createCar(carName, () => callback(createdEngine)) // error here
}

VSCode tells me createdEngine maybe undefined. However this is fine:

  const fn = callback(createdEngine)

  createCar(carname, () => fn)

Is this an intended behaviour?

Answer 1

The 2 cases aren't equivalent.

In the second case you call callback inside the function, in the first you defer the call for a later time.

This will fail with the same error

 const fn = () => callback(createdEngine)

 createCar(carname, fn)

This is related to the fact that TS cannot know what happens with createdEngine because the function will be called later on. When you call createEngine inside your function then it knows that the calls are done in sync so it narrows the type down.

Interesting enough if you ensure that the type of createdEngine is Engine at all times then this will work.

function createCarWithEngine(carName: string, engineName: string, callback: (param: Engine) => void) {
  let createdEngine = createdEngines.find((engine) => engine.name === engineName) || buildEngine(engineName)

  createCar(carName, () => callback(createdEngine)) // error here
}