Run async tasks one after another

Promise.all starts everything at once, which is usually what you want and occasionally a disaster — think four PUT requests to the same record, or an endpoint that returns 429 the moment you exceed one call at a time.

Implement runSequentially(tasks), where tasks is an array of zero-argument functions that each return a promise. Call the first task, wait for it to settle, then call the next, and so on. Resolve with an array of the results in the same order as tasks. An empty array resolves with an empty array. If a task rejects, reject immediately with that same reason and do not call any of the remaining tasks.

const make = (value, ms) => () =>
  new Promise((resolve) => setTimeout(() => resolve(value), ms));

const results = await runSequentially([make("a", 20), make("b", 5)]);
// results === ["a", "b"] — "b" waits for "a", even though it's faster

try {
  await runSequentially([
    async () => 1,
    async () => { throw new Error("stop"); },
    async () => 3, // never runs
  ]);
} catch (error) {
  // error.message === "stop"
}