Timeout Cancellation - Solution & Explanation

Problem Overview: You need to schedule a function fn to run after t milliseconds and return another function that cancels the scheduled execution. If the cancel function runs before the timer fires, fn must never execute. This problem checks your understanding of timer APIs and basic asynchronous programming.

Approach 1: Using setTimeout and clearTimeout (O(1) time, O(1) space)

The direct solution schedules the callback using setTimeout and stores the returned timer identifier. JavaScript timers return an ID that uniquely represents the scheduled task. The cancellation function simply calls clearTimeout(timerId), which removes the scheduled callback from the event loop queue before it executes.

The key insight is that timer scheduling and cancellation are constant‑time operations managed by the runtime. No iteration or extra data structures are required. When the cancel function executes before the delay expires, the timer is invalidated and the callback never runs. If the timer already fired, calling clearTimeout has no effect.

This approach mirrors how real systems manage scheduled tasks. Most interviewers expect you to know the relationship between setTimeout and clearTimeout and how the event loop handles deferred execution in JavaScript.

Approach 2: Using Promises for Asynchronous Control (O(1) time, O(1) space)

Another implementation wraps the delayed execution inside a Promise. A timer triggers the promise resolution after t milliseconds, which then executes the callback. The cancellation function prevents the resolution by clearing the timer or toggling a flag that stops the callback from running.

This design is useful when your codebase already relies heavily on promises or async workflows. For example, a promise-based wrapper can integrate with async/await, cancellation tokens, or higher-level task management systems. The runtime cost remains constant because only one timer and one promise object are created.

While slightly more abstract, this version demonstrates how timers can coordinate with Promises to control asynchronous flow. The core mechanism still relies on canceling the scheduled timeout before execution.

Recommended for interviews: The setTimeout + clearTimeout approach is the expected solution. It shows you understand how timers work in the event loop and how scheduled callbacks can be canceled. The promise-based version is useful for system design discussions or async-heavy applications, but the direct timer cancellation approach is clearer and closer to how the runtime APIs are designed.