48. What is promise.all?

Promise.all() is a method in JavaScript that takes an array (or iterable) of promises and returns a single promise that resolves when all of the promises in the array have been resolved, or rejects if any one of the promises rejects.

Syntax

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Promise.all([promise1, promise2, promise3, ...])

How It Works

1. If all promises resolve, Promise.all() returns a promise that resolves with an array of results (in the same order as the input promises).
2. If any promise rejects, Promise.all() immediately rejects with the reason from the first rejected promise.

Example 1: All promises resolve

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const promise1 = Promise.resolve(10);
const promise2 = Promise.resolve(20);
const promise3 = new Promise((resolve) => setTimeout(() => resolve(30), 1000));

Promise.all([promise1, promise2, promise3])
  .then((results) => {
    console.log(results); // Output: [10, 20, 30]
  })
  .catch((error) => {
    console.log("Error:", error);
  });

✅ All promises resolve → The output is an array of results [10, 20, 30].

Example 2: One promise rejects

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const promise1 = Promise.resolve(10);
const promise2 = Promise.reject("Failed");
const promise3 = new Promise((resolve) => setTimeout(() => resolve(30), 1000));

Promise.all([promise1, promise2, promise3])
  .then((results) => {
    console.log(results);
  })
  .catch((error) => {
    console.log("Error:", error);
  });

❌ Since promise2 rejects, Promise.all() immediately rejects with "Failed" — the other promises' results are ignored.

Example 3: Handling mixed resolved and rejected promises

You can use Promise.allSettled() if you want to handle both resolved and rejected promises:

javascript
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const promise1 = Promise.resolve(10);
const promise2 = Promise.reject("Failed");
const promise3 = new Promise((resolve) => setTimeout(() => resolve(30), 1000));

Promise.allSettled([promise1, promise2, promise3])
  .then((results) => {
    console.log(results);
  });

Output:

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[
  { status: 'fulfilled', value: 10 },
  { status: 'rejected', reason: 'Failed' },
  { status: 'fulfilled', value: 30 }
]

✅ Promise.allSettled() returns the status of each promise, regardless of success or failure.