Sometimes, we want to fix the error ‘”TS2322: Type ‘Timeout’ is not assignable to type ‘number’” when running unit tests with TypeScript.

In this article, we’ll look at how to fix the error ‘”TS2322: Type ‘Timeout’ is not assignable to type ‘number’” when running unit tests with TypeScript.

How to fix the error ‘”TS2322: Type ‘Timeout’ is not assignable to type ‘number’” when running unit tests with TypeScript?

To fix the error ‘”TS2322: Type ‘Timeout’ is not assignable to type ‘number’” when running unit tests’ with TypeScript, we can define the type of the value returned by setTimeout.

For instance, we write

let timeoutId: null | ReturnType<typeof setTimeout> = null;

timeoutId = setTimeout(() => {
  //...
});

to set the timeoutId to the null | ReturnType<typeof setTimeout> union type.

We use typeof setTimeout to get the type for the setTimeout function.

And we use ReturnType to get the return type of the setTimeout function.

Then we can assign the value returned by setTimeout to timeoutId without error.

Conclusion

To fix the error ‘”TS2322: Type ‘Timeout’ is not assignable to type ‘number’” when running unit tests’ with TypeScript, we can define the type of the value returned by setTimeout.