Rate limit with a sliding window

Public APIs publish limits like 'ten requests per second' and answer with 429 when you exceed them. A client-side limiter keeps you under the line without you having to think about it at every call site.

Implement createRateLimiter(limit, windowMs), returning a schedule(task) function. task takes no arguments and returns a promise; schedule returns a promise that settles with that task's own result. At most limit tasks may be **started** within any windowMs window. A task that cannot start yet waits, and waiting tasks start in the order they were scheduled. Once a start falls further than windowMs in the past it no longer counts against the limit, so a slot frees up. A task that rejects only rejects its own promise and does not hold up the ones behind it.

What it has to do

• Start at most limit tasks within any windowMs window.
• Start the first limit tasks without any delay.
• Start queued tasks in the order they were scheduled.
• Settle each returned promise with its own task's result.
• Let a rejecting task fail alone, without stalling the queue.

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