IconVault's requestIdleCallback Playground teaches cooperative scheduling by running the real API in your browser. Configure the task count, work per task, the timeout option, and simulated main-thread load, then watch each idle callback execute with its actual deadline.timeRemaining() budget, wait time, and runtime drawn as a live bar timeline. Tasks respect their budget like production code should, and a CSV export captures every measurement. It is free and runs fully in your browser.

requestIdleCallback is how you schedule non-urgent work (analytics, prefetching, cleanup) without janking the page. This lab makes the abstract concrete: you see callbacks wait for genuine idle time, fire early with didTimeout=true when the timeout beats the idle window, and shrink under load. On browsers without the API (Safari), it falls back to setTimeout and marks every row honestly, so the lesson survives the platform gap.