Typescript Debounce

debounce.ts

// ts 3.6x
function debounce<T extends Function>(cb: T, wait = 20) {
    let h = 0;
    let callable = (...args: any) => {
        clearTimeout(h);
        h = setTimeout(() => cb(...args), wait);
    };
    return <T>(<any>callable);
}

// usage
let f = debounce((a: string, b: number, c?: number) => console.log(a.length + b + c || 0));
f("hi", 1, 1);
f("world", 1);

I've been trying to use ThisParameterType without success. I don't see how you can declare a function wrapper in TypeScript that preserves the context type.

1. this derives from the function being called, not the function parameter.
2. Arrow functions don't bind a reference to this, so convert it to a function.

function debounce<T extends (...args: Parameters<T>) => void>(this: ThisParameterType<T>, fn: T, delay = 300) {
  let timer: ReturnType<typeof setTimeout> | undefined
  return (...args: Parameters<T>) => {
    clearTimeout(timer)
    timer = setTimeout(() => fn.apply(this, args), delay)
  }
}

Still doesn't work:

Playground

compared to:

Playground

Still doesn't work:

Playground

compared to:

Playground

I think typescript can't infer function say‘ this, so try this: function say(this: {name: string,skeak: any}),

What about this ?

export function debounce<T extends (...args: any[]) => any>(
  this: any,
  fn: T,
  wait: number
): (...args: Parameters<T>) => void {
  let timer: ReturnType<typeof setTimeout>;

  return (...args: Parameters<T>) => {
    if (timer) {
      clearTimeout(timer); // clear any pre-existing timer
    }
    const context: any = this; // get the current context
    timer = setTimeout(() => {
      fn.apply(context, args); // call the function if time expires
    }, wait);
  };
}

worked for me this one

export function debounce<T extends (...args: any[]) => any>(cb: T, wait: number) {
  let h: any;
  const callable = (...args: any) => {
      clearTimeout(h);
      h = setTimeout(() => cb(...args), wait);
  };
  return <T>(<any>callable);
}

I updated mine to use unknown instead of any because otherwise TS complains even though it doesn't matter in this particular case since we're not reassigning args <T extends (...args: unknown[]) => unknown>.

Another version,

• it respects the types (parameters and return type) of the function to be debounced
• it was made to be used for debouncing functions that returns something (and avoids side-effects). Result is wrapped in a promise. Consequently you'll have to await the debounced function.

The trick here (TS >=v4.6), is the generic <T extends (...args: Parameters<T>) => ReturnType<T>>:

export function debounce<T extends (...args: Parameters<T>) => ReturnType<T>>(
	callback: T,
	delay: number
) {
	let timer: ReturnType<typeof setTimeout>;
	return (...args: Parameters<T>) => {
		const p = new Promise<ReturnType<T> | Error>((resolve, reject) => {
			clearTimeout(timer);
			timer = setTimeout(() => {
				try {
					let output = callback(...args);
					resolve(output);
				} catch (err) {
					if (err instanceof Error) {
						reject(err);
					}
					reject(new Error(`An error has occurred:${err}`));
				}
			}, delay);
		});
		return p;
	};
}

The correct type for timeout is ReturnType<typeof setTimeout> not number

The return value is a positive integer which identifies the timer created by the call to setTimeout(). In the browser correct call is window?.setTimeout().

https://developer.mozilla.org/en-US/docs/Web/API/setTimeout#return_value

Still no solution that solves my context issue

 function debounce<F extends (...args: Parameters<F>) => ReturnType<F>>(
  func: F,
  waitFor: number,
): (...args: Parameters<F>) => void {
  let timeout: ReturnType<typeof setTimeout>;
  
  return (...args: Parameters<F>): void => {
    clearTimeout(timeout);
    timeout = setTimeout(() => func(...args), waitFor);
  };
}

const person = {
  name: 'Jack',
  speak: function say(sec:number){
  debounce(()=>console.log(`My name is ${this.name} , debounced ${sec}`),sec)();
  }

}

person.speak(3_000);
debounce(()=>console.log('hehe'),100)();
debounce(()=>console.log('hehe 1_000'),1_000)();
person.speak(4_000);

not sure if this solution addresses the issue but works

playground

You create a new function every time, therefore person.speak is not debounced

@sylvainpolletvillard I landed here by chance and your initial example works fine for me when I simply ignore the Typescript error commenting the lines before 'this' - not sure if that is a valid solution for you though:

// @ts-expect-error: ignore

Sure, ignoring the error works to fix the error

 debounce = <F extends (...args: Parameters<F>) => ReturnType<F>>(
  func: F,
  waitFor: number,
) => {
  let timeout: NodeJS.Timeout

  const debounced = (...args: Parameters<F>) => {
    clearTimeout(timeout)
    timeout = setTimeout(() => func(...args), waitFor)
  }

  return debounced
}

The timeout type should be ReturnType<typeof setTimeout>