Implement Debounce

Problem Context

In modern web applications, certain events (like window resizing, search input, or button clicks) can fire dozens of times per second. Executing expensive operations on every single event is highly inefficient. A debounce function solves this by ensuring that a given function is only called after a specified delay has elapsed since the last time it was invoked.

Task

Implement a debounce(fn, delay) function from scratch in JavaScript. The debounced function should:

• Delay the execution of fn by delay milliseconds after the last call.
• If the debounced function is called again before the delay expires, reset the timer.
• Return a new function (the debounced version) that, when invoked, schedules fn to be called with the correct arguments and this context after the delay.

Input / Output Format

Input:

• fn — A function to debounce.
• delay — A non-negative integer representing the delay in milliseconds.

Output:

• Returns a new debounced function. When this debounced function is invoked multiple times rapidly, fn is only called once after delay ms have passed since the last invocation.

Examples

Example 1 — Basic Debounce

If a debounced function with delay = 300 is called at t=0ms and t=100ms, fn should only execute once at approximately t=400ms (100ms + 300ms).

Example 2 — Single Call

If the debounced function is called only once, fn executes after delay ms with the correct arguments.

Example 3 — Rapid Calls

If the debounced function is called 5 times rapidly in succession (within the delay window), fn executes only once — after the delay from the last call.

Approach Hints

1. Closure + Timer: Use a closure to maintain a reference to a timerId (the return value of setTimeout).
2. Clear and Reset: Each time the debounced function is called, use clearTimeout(timerId) to cancel any pending timer, then setTimeout to schedule a new one.
3. Preserve Context and Arguments: Use fn.apply(this, arguments) inside the setTimeout callback to ensure the original function receives the correct this context and all arguments.
4. Edge Case — Zero Delay: A delay of 0 should still defer execution asynchronously (to the end of the current event loop tick).

Function Signature

/**
 * @param {Function} fn
 * @param {number} delay
 * @return {Function}
 */
function debounce(fn, delay) {
  // Your implementation here
}