How Promise.all Works in a Single-Threaded JavaScript World

If JavaScript is single-threaded, how can Promise.all() handle multiple requests at the same time?

The short answer:

JavaScript is single-threaded for executing code, but I/O is handled outside the JS thread.

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🧠 What “single-threaded” really means

• Only one piece of JavaScript runs at a time.
• JS does NOT block while waiting for network, timers, or disk.

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⚙️ Step-by-step: What happens with Promise.all

await Promise.all([
  fetch("/a"),
  fetch("/b"),
  fetch("/c"),
]);

1️⃣ Requests start immediately

const p1 = fetch("/a");
const p2 = fetch("/b");
const p3 = fetch("/c");

• Promises are created instantly.
• Network work starts in the background.
• JS thread is free.

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2️⃣ Network runs outside JS

• Browser or Node runtime handles requests.
• Requests run concurrently.
• JS thread does nothing during this time.

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3️⃣ Promise resolution uses microtasks

• When a request finishes, its Promise resolves.
• .then() callbacks go to the microtask queue.
• JS executes them one by one.

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4️⃣ Promise.all resolves last

• Waits for all Promises to resolve.
• Rejects early if one fails.

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🔄 Timeline

JS Thread:
| start A | start B | start C | free | handlers |

Network:
|---- A ----|
|------ B ------|
|-- C --|

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🧠 Mental Model

• JS thread = single chef
• Network = multiple delivery drivers
• Promise.all = wait for all deliveries

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⚠️ Important note

Promise.all() does NOT make CPU work parallel.

Promise.all([
  heavyTask(),
  heavyTask(),
]);

This still blocks the UI.

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🧱 Browser vs Node

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✅ Summary

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✨ Final takeaway

JS coordinates work — it doesn’t do it all itself.
Promise.all works because the heavy work happens outside the JS thread.