Fall back when a request is too slow

Arguments

valuePromise<T> | T

The primary promise or value.

fallbackT

The value to use if value is too slow or rejects.

msnumber

How long to wait for value before using the fallback.

Returns

Promise<T> — A promise that resolves with value's result if it arrives within ms, otherwise with fallback. Never rejects.

Examples

const live = new Promise((resolve) => setTimeout(() => resolve("live"), 5));
const result = await withFallback(live, "cached", 100);
// result === "live" — it arrived in time

const slow = new Promise((resolve) => setTimeout(() => resolve("live"), 400));
const result = await withFallback(slow, "cached", 30);
// result === "cached" — the timer fired first

const failing = Promise.reject(new Error("offline"));
failing.catch(() => {});
const result = await withFallback(failing, "cached", 100);
// result === "cached" — rejection also triggers the fallback

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.

Notes

• Race the input against a timer that resolves with the fallback, and .catch() the input to the fallback as well — one Promise.race handles both branches.
• The returned promise never rejects, so callers never need a try/catch.