Cannot use Fake Timer to test setTimeout for class method-CodePudding

I can test setTimeout with a normal function like this:

function doAsync() {
  setTimeout(doAsync, 1000)
}

jest.useFakeTimers()

test("setTimeout with function", async () => {
  doAsync()
  jest.advanceTimersByTime(2500)
  expect(setTimeout).toBeCalledTimes(3)
})

But when I use it with class method, it cannot refer to this, Cannot read properties of null (reading 'doAsync'):

class Test {
  doAsync() {
    setTimeout(this.doAsync, 1000)
  }
}

jest.useFakeTimers()

test("setTimeout with class method", async () => {
  new Test().doAsync()
  jest.advanceTimersByTime(2500)
  expect(setTimeout).toBeCalledTimes(3)
})

How can I test with a class method like this?

CodePudding user response：

You need to write :

class Test {
  doAsync() {
    setTimeout(() => this.doAsync(), 1000)
  }
}

It's because when you write this.doAsync, at the moment of the setTimeout execution, this does exist anymore.

But if round it up around a lambda, the this reference is copied in it at his declaration and so it reference the good instance when it's called later.

Hope it's helps.

CodePudding user response：

Wrap this.doAsync call into an arrow function to preserve the reference to this

class Test {
    doAsync() {
        setTimeout(() => this.doAsync(), 1000)
    }
}