Write debounce from scratch

Debouncing controls how often a function can execute over time. When a JavaScript function is debounced with a wait time of wait milliseconds, it runs only after wait milliseconds have elapsed since the debounced function was last called.

You have probably encountered debouncing in daily life before, such as when entering an elevator. Only after some time passes without pressing the 'Door open' button (the debounced function not being called) will the elevator door actually close (the callback function is executed).

Implement debounce(fn, wait) so that fn is called only after wait milliseconds have passed since the most recent call. The returned function should not invoke fn immediately. When the delayed call finally runs, it should use the latest arguments and preserve the this value from the most recent call.

let i = 0;
function increment() { i++; }
const debouncedIncrement = debounce(increment, 100);

// t = 0: nothing happens yet
debouncedIncrement();

// t = 50: still waiting
// t = 100: increment runs once, i === 1

let value = null;
const update = (v) => { value = v; };
const debounced = debounce(update, 100);

debounced("j");
debounced("ja");
debounced("jav");
// 100ms later: value === "jav"

const d = debounce(() => console.log("fired"), 100);
d();
d.cancel(); // nothing prints — the pending call was dropped