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Timeout Cancellation

EasyUpdated: Aug 2, 2025
Practice on:
LeetCode

Problem

Given a function fn, an array of arguments args, and a timeout t in milliseconds, return a cancel function cancelFn.

After a delay of cancelTimeMs, the returned cancel function cancelFn will be invoked.

setTimeout(cancelFn, cancelTimeMs)

Initially, the execution of the function fn should be delayed by t milliseconds.

If, before the delay of t milliseconds, the function cancelFn is invoked, it should cancel the delayed execution of fn. Otherwise, if cancelFn is not invoked within the specified delay t, fn should be executed with the provided args as arguments.

Examples

Example 1

Input: fn = (x) => x * 5, args = [2], t = 20
Output: [{"time": 20, "returned": 10}]
Explanation: 
const cancelTimeMs = 50;
const cancelFn = cancellable((x) => x * 5, [2], 20);
setTimeout(cancelFn, cancelTimeMs);

The cancellation was scheduled to occur after a delay of cancelTimeMs (50ms), which happened after the execution of fn(2) at 20ms.

Example 2

Input: fn = (x) => x**2, args = [2], t = 100
Output: []
Explanation: 
const cancelTimeMs = 50;
const cancelFn = cancellable((x) => x**2, [2], 100);
setTimeout(cancelFn, cancelTimeMs);

The cancellation was scheduled to occur after a delay of cancelTimeMs (50ms), which happened before the execution of fn(2) at 100ms, resulting in fn(2) never being called.

Example 3

Input: fn = (x1, x2) => x1 * x2, args = [2,4], t = 30
Output: [{"time": 30, "returned": 8}]
Explanation: const cancelTimeMs = 100;
const cancelFn = cancellable((x1, x2) => x1 * x2, [2,4], 30);
setTimeout(cancelFn, cancelTimeMs);

The cancellation was scheduled to occur after a delay of cancelTimeMs (100ms), which happened after the execution of fn(2,4) at 30ms.

Constraints

  • fn is a function
  • args is a valid JSON array
  • 1 <= args.length <= 10
  • 20 <= t <= 1000
  • 10 <= cancelTimeMs <= 1000

Solution

Method 1 – setTimeout and clearTimeout

Intuition

Use setTimeout to schedule the function, and return a cancel function that calls clearTimeout.

Approach

Call setTimeout to schedule fn(...args) after t ms, and return a function that cancels the timeout.

Code

JavaScript
function cancellable(fn, args, t) {
    const timeoutId = setTimeout(() => fn(...args), t);
    return function cancelFn() {
        clearTimeout(timeoutId);
    };
}

Complexity

  • ⏰ Time complexity: O(1)
  • 🧺 Space complexity: O(1)

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