Write sleep() with promises

Sooner or later you need to wait — before a retry, between polling attempts, or to let an animation finish. setTimeout alone does not compose with await, so teams wrap it once and reuse it everywhere.

Implement sleep(ms). It returns a Promise that resolves with undefined after at least ms milliseconds have passed, and it must never reject. sleep(0) still resolves asynchronously — code that runs after the call but before the promise settles must run first. Nothing about the page should be blocked while waiting, so a busy loop is not a solution.

What it has to do

• Return a real Promise instance, even for ms of 0.
• Resolve after at least ms milliseconds, never before.
• Resolve asynchronously: any code that follows the call synchronously runs first.
• Several sleeps started at once must resolve in order of their durations.

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