Published on June 22, 2024

Return type of setTimeout and setInterval in JavaScript

While working on a TypeScript project, I had to save a reference to the setTimeout function. That’s when I needed to know the return type of the setTimeout function in JavaScript. There are three different ways to get the return type of the setTimeout and setInterval functions in JavaScript.

Using :number

Normally, the return type of the above functions is just a number. This number is the ID of the timer that can be used to clear the timer using the clearTimeout or clearInterval functions.

let timer: number;

//....

timer = setTimeout(() => {
	console.log('Hello, World!');
}, 1000);

Using NodeJS.Timeout

The first thing GitHub Copilot suggested to me was to use the NodeJS.Timeout type. This type is provided by the @types/node package and is a more specific type for the return value of the setTimeout function.

let timer: NodeJS.Timeout;

//....

timer = setTimeout(() => {
	console.log('Hello, World!');
}, 1000);

However, I’m not sure if this type is available in the browser environment and whether it is a number or a node-specific type.

So, here’s the most generic way to get the return type of the setTimeout function.

Using ReturnType<typeof setTimeout>

The ReturnType utility type, provided by TypeScript, extracts the return type of a function. We can use this utility type to get the return type of the setTimeout function.

let timer: ReturnType<typeof setTimeout>;

//....

timer = setTimeout(() => {
	console.log('Hello, World!');
}, 1000);

This way, we can get the return type of the setTimeout function in JavaScript, no matter if we are working in a Node.js environment or a browser environment.

Conclusion

Even though there are multiple ways to get the return type of the above functions, I recommend using the ReturnType<typeof setTimeout> method as it is the most generic way.

Something wrong or just found a typo? Edit this page on GitHub and make a PR!

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