Run Promises in sequence

Start each task after the previous one finishes and collect the results in order.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

async function runInSequence(tasks) {
  const results = [];
  for (const task of tasks) results.push(await task());
  return results;
}

Usage

const log = [];
const step = (name, ms) => () => new Promise(resolve => setTimeout(() => { log.push(name); resolve(name); }, ms));
await runInSequence([step('slow', 30), step('fast', 10)]); // → ['slow', 'fast']
log; // → ['slow', 'fast']
// Promises, unlike tasks, are already running:
log.length = 0;
await Promise.all([step('slow', 30)(), step('fast', 10)()]); // → ['slow', 'fast']
log; // → ['fast', 'slow']

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Pass functions that start the work, not promises: a promise is already running once it exists. The original took promises, so it ordered the results but not the work, as the last lines show.
• Use it when each step must finish before the next starts, such as sending one batch at a time under a rate limit. On a 429, wait for Retry-After (see Read a Retry-After header) before the next task.
• The first rejection stops the sequence; catch inside each task to keep going.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Convert Celsius to Fahrenheit — Apply the temperature conversion without rounding the result.
• Convert hex to RGB — Expand #rgb if needed and read the red, green and blue channels.
• Convert URL parameters to object — Read a query string into an object; repeated keys become arrays.