Fall back when a request is too slow

A widget that waits three seconds for fresh numbers feels broken. Showing slightly stale data after 300ms feels instant, and the fresh data can arrive later.

Implement withFallback(value, fallback, ms). It returns a promise that resolves with the resolved value of value if that happens within ms milliseconds. If ms elapses first, resolve with fallback. If value rejects before ms elapses, also resolve with fallback. A non-promise value is treated as already fulfilled. The returned promise must never reject.

What it has to do

• Resolve with the input's value when it fulfils within ms.
• Resolve with fallback when ms elapses first.
• Resolve with fallback when the input rejects.
• Accept a plain value as well as a promise.
• Never reject.

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