How do I replace the setTimeout method in a typesafe manner?

November 23, 2023

In my tests I want to replace the setTimeout method so that the tests run fast.

Originally I had the equivalent piece of code that used "any"… But of course eslint etc bleats.

I now have this piece of code which at last does not have any in it.

  let defaultTimeout: typeof globalThis.setTimeout;
  beforeEach(() => {
    defaultTimeout = global.setTimeout;
    globalThis.setTimeout = (callback: TimerHandler) => { (1)
      defaultTimeout(callback, 500);
    };
  })

However, I still get an error at globalThis.setTimeout

TS2741: Property promisify is missing in type (callback:
TimerHandler) => void but required in type typeof setTimeout
timers.d.ts(161, 19): promisify is declared here.

I know that I can resolve this by using any and unknown… is there another way?

>Solution :

Overriding functions like setTimeout, setInterval, console.* is bad practice. Why do you do it in that way? Did you consider using timer mocks?

I assume you use Jest or Vitest, anyway the logic is identical.

If all you want is to skip the timers, you could do

jest.useFakeTimers();

// Your test code goes here...

expect(true).toEqual(true);

jest.runAllTimers();

jest.useRealTimers();

The jest.runAllTimers(); line is the one doing the trick, "fast-forwarding" the timers, read more here: https://jestjs.io/docs/jest-object#jestrunalltimers