Level 5: Async and functions · Exercise 71 of 76 · Free

Throttle

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Return a function that runs `fn` at most once every `wait` milliseconds, on the leading edge. The first call runs `fn` at once with the call's arguments and returns its result. A call that arrives less than `wait` ms after the last run is dropped and returns `undefined`. A call exactly `wait` ms (or more) after the last run runs again. There is no trailing call for dropped calls. Compare times with `Date.now()`. Tests run on a fake clock: `setTimeout`, `clearTimeout` and `Date.now` are simulated, so waits are instant and exact.

What this exercise teaches

Storing the time of the last run and comparing with >= makes a call exactly wait ms later run, which a > comparison drops. Debounce waits for quiet and runs at the end; throttle runs first and ignores the rest of the window.

A hint if you get stuck

Keep `last`, the time of the last run, starting at `-Infinity`. In the returned function read `Date.now()`, and only when `now - last >= wait` update `last` and call `fn` with the arguments.