27Implement throttle() with leading and trailing options

Implement throttle(fn, wait, { leading = true, trailing = true } = {}). The throttled function calls fn at most once per wait milliseconds, however often it is called.

• leading: true calls fn immediately on the first call of a quiet period.
• trailing: true makes sure the last call made during a waiting period is not lost: fn runs with its arguments when the period ends.

Also add cancel(), which drops any pending trailing call.

const onScroll = throttle(() => updateProgressBar(), 100);
window.addEventListener("scroll", onScroll);

Requirements

• Call fn with the arguments and this of the call that triggered it.
• A trailing call starts a new waiting period, so two calls are never closer than wait apart.
• With leading: false, the first call only schedules a trailing call.
• If nothing was called during a waiting period, no trailing call happens.