Practical patterns for sequencing, parallelizing, error-handling, and hardening async flows — without the pyramid of doom.

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Introduction

Promises are the backbone of modern JavaScript async. But the how — sequencing multiple operations, combining results, handling errors cleanly, adding timeouts/retries, limiting concurrency — still trips up even seasoned devs.

This guide is a hands-on playbook. You’ll learn exactly how to chain Promises for real projects: from simple “do A then B” to “kick off 50 calls with a pool of 5,” plus rock-solid error handling, cancellation, and production safeguards. I’ll keep it conversational, with copy-paste snippets and gotchas highlighted.

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1) Promise Chaining in 90 Seconds

• A Promise represents an async result that will either fulfill (resolve) or reject.
• .then(onFulfilled, onRejected) returns a new Promise. Whatever you return from onFulfilled (or onRejected) becomes the next Promise’s value. Throwing an error rejects the next promise.
• .catch(onRejected) is sugar for .then(undefined, onRejected).
• .finally(cb) runs regardless of success/failure; it does not receive the value—use it for cleanup.

Core mental model:

doA()
  .then(resultA => doB(resultA))   // return → chains to next
  .then(resultB => doC(resultB))   // each step returns a promise or value
  .catch(err => handle(err))       // any prior rejection lands here
  .finally(() => cleanup());       // runs either way

Rule: Always either return a value/promise or throw inside .then/.catch. Silent fallthrough = surprise bugs.

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2) Sequencing: Waterfalls Without Tears

Pattern: Linear chain

fetchUser(id)
  .then(user => getProfile(user))
  .then(profile => enrich(profile))
  .then(enriched => save(enriched))
  .then(saved => notify(saved.email))
  .catch(reportError);

Equivalent with async/await (same semantics)

try {
  const user     = await fetchUser(id);
  const profile  = await getProfile(user);
  const enriched = await enrich(profile);
  const saved    = await save(enriched);
  await notify(saved.email);
} catch (err) {
  reportError(err);
}

When to use: Each step depends on the previous result (e.g., auth → fetch → transform → persist).

Gotcha (classic): Forgetting to return a promise inside .then:

// ❌ WRONG: inner promise is unchained; next then runs too early
doA().then(a => { doB(a); }).then(...);

// ✅ RIGHT:
doA().then(a => { return doB(a); }).then(...);
// or concise:
doA().then(doB).then(...);

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3) Parallelism: Finish Faster with Promise.all

Start independent work first, then await together.

const userP   = fetchUser(id);
const postsP  = fetchPosts(id);
const prefsP  = fetchPreferences(id);

Promise.all([userP, postsP, prefsP])
  .then(([user, posts, prefs]) => render({ user, posts, prefs }))
  .catch(reportError);

Why this rocks: True concurrency for I/O-bound tasks. The slowest promise determines total time.

Fail-fast behavior: If any member rejects, Promise.all rejects immediately and cancels nothing (JS promises have no automatic cancellation). Use AbortController if you must abort in-flight work (see §10).

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4) Soft-Fail Aggregation with Promise.allSettled

When you want all outcomes, regardless of failures:

const outcomes = await Promise.allSettled(urls.map(fetchJson));
for (const o of outcomes) {
  if (o.status === "fulfilled") use(o.value);
  else log(o.reason);
}

Use case: Bulk jobs, dashboards, where partial results still matter.

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5) Who Wins First? Promise.race vs Promise.any

• Promise.race([p1, p2, ...]) settles with the first to settle (fulfilled or rejected).
• Promise.any([p1, p2, ...]) settles with the first to fulfill (ignores rejections until all reject; then throws AggregateError).

// race: first settled wins (use for timeouts)
Promise.race([
  fetch(url),
  timeout(3000) // a rejecting promise
]).then(handle).catch(report);

// any: first success wins
Promise.any(mirrors.map(m => fetchFromMirror(m)))
  .then(useFastest)
  .catch(e => console.error('All mirrors failed', e.errors));

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6) Composition: Promise-Powered Pipelines

Make small functions that return a promise/value. Then compose.

const parseJSON = (r) => r.json();
const ensure200 = (r) => r.ok ? r : Promise.reject(new Error('HTTP ' + r.status));
const pickName  = (u) => u.name;

fetch('/api/user/42')
  .then(ensure200)
  .then(parseJSON)
  .then(pickName)
  .then(console.log)
  .catch(reportError);

Tip: If a step might throw synchronously, that throw automatically becomes a rejection in the chain. No try/catch needed here—handle at the end.

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7) Branching Chains Without Duplicate Code

Pattern: Conditional continuation

getOrder(id)
  .then(order => {
    if (order.status === 'PAID') {
      return fulfill(order);         // chain continues with fulfill’s promise
    } else {
      return charge(order).then(() => fulfill(order));
    }
  })
  .then(sendReceipt)
  .catch(reportError);

Pattern: Early exit (resolve to a sentinel)

getFeatureFlag('beta')
  .then(flag => flag ? loadBeta() : 'SKIPPED')
  .then(result => {
    if (result !== 'SKIPPED') initBetaUI(result);
  });

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8) Concurrency Limits: Don’t DDOS Your Backend

Goal: Kick off many tasks but only N at a time.

Tiny pool utility

function limit(fn, pool = 5) {
  let active = 0, queue = [];
  const run = (...args) => new Promise((resolve, reject) => {
    const task = async () => {
      active++;
      try    { resolve(await fn(...args)); }
      catch  (e) { reject(e); }
      finally {
        active--;
        if (queue.length) queue.shift()();
      }
    };
    active < pool ? task() : queue.push(task);
  });
  return run;
}

Use it:

const fetchLimited = limit(fetchJson, 5);
const results = await Promise.all(urls.map(u => fetchLimited(u)));

Why this matters: APIs have rate limits; your users have bandwidth/CPU limits.

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9) Production Hardening: Timeouts, Retries, Backoff

Timeout wrapper

const withTimeout = (p, ms, msg = 'Timeout') =>
  new Promise((res, rej) => {
    const t = setTimeout(() => rej(new Error(msg)), ms);
    p.then(v => { clearTimeout(t); res(v); },
           e => { clearTimeout(t); rej(e); });
  });

Exponential backoff retry

async function retry(fn, { retries = 3, base = 200, factor = 2 } = {}) {
  let attempt = 0, last;
  while (attempt++ <= retries) {
    try { return await fn(); }
    catch (e) {
      last = e;
      if (attempt > retries) break;
      await new Promise(r => setTimeout(r, base * factor ** (attempt - 1)));
    }
  }
  throw last;
}

Combine them:

await retry(() => withTimeout(fetchJson(url), 4000), { retries: 2, base: 300 });

Pro tip: Fail fast on non-retryable errors (e.g., 4xx). Only retry transient errors (5xx, network).

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10) Cancellation: Promises Don’t Cancel — Your Code Should

Use AbortController with APIs that support it (fetch, many libs).

const controller = new AbortController();
const p = fetch(url, { signal: controller.signal }).then(r => r.json());

// later (user navigates away):
controller.abort(); // rejects with DOMException: "AbortError"

In chains: pass the same signal through, and check signal.aborted where needed. If a race finishes early, abort the losers:

const c1 = new AbortController();
const c2 = new AbortController();

const fast = Promise.any([
  fetch(mirror1, { signal: c1.signal }),
  fetch(mirror2, { signal: c2.signal })
]);

fast.finally(() => { c1.abort(); c2.abort(); }); // cleanup losers

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11) Error Handling: Centralized, Informative, Fair

Single .catch for linear chains

doA()
  .then(doB)
  .then(doC)
  .catch(err => {
    console.error('Pipeline failed', err);
    // decide to swallow, rethrow, or transform
    throw err; // or return a fallback
  });

Localized recovery

fetchJson(url)
  .catch(e => (e.status === 404 ? {} : Promise.reject(e)))
  .then(render);

Preserve context

.doSomething()
.then(...)
.catch(e => {
  e.context = { userId, url };
  throw e;
})
.catch(reportWithContext);

finally for cleanup

connect()
  .then(conn => query(conn).finally(() => conn.close()))
  .then(useResults)
  .catch(reportError);

Remember: finally doesn’t receive the value; it passes through the prior outcome (unless it throws).

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12) Common Gotchas (and Fixes)

❌ Forgetting return inside .then

doA().then(a => { doB(a); }).then(use); // use runs too early
// ✅
doA().then(a => doB(a)).then(use);

❌ Using forEach with async

// forEach ignores returned promises
urls.forEach(async u => await fetch(u)); // tasks fire, but you can't await completion
// ✅ Use map + Promise.all:
await Promise.all(urls.map(u => fetch(u)));

❌ Accidental serialization

for (const u of urls) await fetch(u); // sequential.
// ✅
await Promise.all(urls.map(u => fetch(u)));

❌ Swallowing errors in .catch

doA().catch(e => console.log(e)).then(stepB); // stepB runs even if A failed
// ✅ Rethrow or return a fallback intentionally:
doA().catch(e => { log(e); throw e; }).then(stepB);

❌ Starving the loop with microtasks

Creating enormous promise chains that resolve synchronously can starve timers/UI. Break it up:

await Promise.resolve(); // yield a tick (or queueMicrotask)

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13) Patterns You’ll Reuse

13.1 Map → Chain (transform then act)

fetchJson(url)
  .then(validate)
  .then(normalize)
  .then(save)
  .catch(reportError);

13.2 Reduce → Waterfall (sequential over a list)

await urls.reduce(
  (p, url) => p.then(() => fetchJson(url).then(save)),
  Promise.resolve()
);

13.3 Batch with backpressure

const doLimited = limit(fetchJson, 4);
const results = await Promise.all(items.map(x => doLimited(x)));

13.4 Circuit breaker (simple)

function breaker(fn, { failMax = 5, coolMs = 10_000 } = {}) {
  let fails = 0, openUntil = 0;
  return (...args) => {
    const now = Date.now();
    if (now < openUntil) return Promise.reject(new Error('circuit open'));
    return fn(...args).catch(e => {
      if (++fails >= failMax) { openUntil = now + coolMs; fails = 0; }
      throw e;
    });
  };
}

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14) End-to-End Example: From Callback Pyramid → Clean Chain

Before (callback hell):

getUser(id, (e, user) => {
  if (e) return next(e);
  getPermissions(user, (e, perms) => {
    if (e) return next(e);
    getTeam(user.teamId, (e, team) => {
      if (e) return next(e);
      sendEmail(user.email, team, perms, (e) => {
        if (e) return next(e);
        res.json({ ok: true });
      });
    });
  });
});

After (chained promises):

const p = (fn) => (...args) => new Promise((res, rej) =>
  fn(...args, (e, v) => e ? rej(e) : res(v))
);

const getUserP        = p(getUser);
const getPermissionsP = p(getPermissions);
const getTeamP        = p(getTeam);
const sendEmailP      = p(sendEmail);

getUserP(id)
  .then(user => Promise.all([user, getPermissionsP(user), getTeamP(user.teamId)]))
  .then(([user, perms, team]) => sendEmailP(user.email, team, perms))
  .then(() => res.json({ ok: true }))
  .catch(next);

Readable, parallel where safe, single .catch for centralized error handling.

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15) Quick Reference: Which Combo Do I Use?

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16) Copy-Paste Utilities

export const limit = (fn, pool = 5) => {
  let active = 0, queue = [];
  const run = (...args) => new Promise((resolve, reject) => {
    const task = async () => {
      active++;
      try { resolve(await fn(...args)); }
      catch (e) { reject(e); }
      finally {
        active--;
        if (queue.length) queue.shift()();
      }
    };
    active < pool ? task() : queue.push(task);
  });
  return run;
};

export const withTimeout = (p, ms, msg = 'Timeout') =>
  new Promise((res, rej) => {
    const t = setTimeout(() => rej(new Error(msg)), ms);
    p.then(v => { clearTimeout(t); res(v); },
           e => { clearTimeout(t); rej(e); });
  });

export const retry = async (fn, { retries = 3, base = 200, factor = 2 } = {}) => {
  let attempt = 0, last;
  while (attempt++ <= retries) {
    try { return await fn(); }
    catch (e) {
      last = e;
      if (attempt > retries) break;
      await new Promise(r => setTimeout(r, base * factor ** (attempt - 1)));
    }
  }
  throw last;
};

export const timeout = (ms, msg = 'Timeout') =>
  new Promise((_, rej) => setTimeout(() => rej(new Error(msg)), ms));

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17) Testing Chained Promises (No Flakes)

• Return or await the promise in your test so the runner knows when it’s done.
• Mock network/filesystem; don’t hit real services.
• Use fake timers for timeouts/retries.
• Assert success with resolves, failure with rejects.

await expect(fetchUser(id)).resolves.toMatchObject({ id });
await expect(withTimeout(delay(10), 5)).rejects.toThrow(/Timeout/);

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Conclusion

Chaining promises “like a boss” boils down to a few fundamentals:

• Return results from .then, throw to reject, and keep a single .catch when you can.
• Parallelize independent work with Promise.all; serialize only when there’s a dependency.
• Use the right combiner (all, allSettled, race, any) for the job.
• Add timeouts, retries, cancellation, and concurrency limits for production-grade reliability.
• Prefer small, composable functions that return promises or values; chain them like Lego.

Pro tip: When an async flow misbehaves, say out loud: “What does this .then return? Where does rejection go? Am I starting everything first, or serializing accidentally?” That debug mantra pays dividends.

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Call to Action

What’s your favorite promise pattern — Promise.any for mirrors, a concurrency pool, or a clever race timeout?
💬 Share your snippet in the comments.
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👩‍💻 Send it to a teammate still wrestling with nested callbacks.