87. Advanced Debounce Function

Implement an advanced debounce with cancellation, flushing, and optional immediate mode.

Requirements

1) Signature

• Implement debounce(fn, delay, immediate = false).
• Return a debounced function debounced(...args) with methods:
• debounced.cancel()
• debounced.flush()

2) Trailing mode (immediate = false)

• Calls reset the timer.
• Only the latest call in a burst is eventually invoked.
• debounced returns undefined while work is pending.
• flush() immediately executes the pending call and returns its result.

3) Immediate mode (immediate = true)

• First call in an idle window executes immediately and returns result.
• Calls during the delay window do not execute fn; they only extend the cooldown timer.
• No trailing invocation occurs from those suppressed calls.

4) Utility behavior

• cancel() clears any pending timer and pending invocation state.
• flush() returns undefined when nothing is pending (or when in immediate mode).
• Preserve call-site this and latest arguments when invoking fn.

5) Delay edge case

• With delay = 0, execution must remain asynchronous in trailing mode.

Examples

const d = debounce((x) => x * 2, 50)
d(1)
d(2)
d(3)
// after ~50ms => fn runs once with 3

const d = debounce((x) => x + 1, 100)
d(4)
d.flush() // 5 (runs immediately)

const d = debounce((x) => x, 100, true)
d("a") // "a" (immediate)
d("b") // undefined (suppressed)