Promise.all for parallel execution in Node.js

Introduction

Promise.all helps you run many tasks at the same time and wait for all of them to finish. This makes your program faster and more efficient.

When you want to fetch data from multiple websites at once.

When you need to read several files from disk in parallel.

When you want to run multiple independent calculations together.

When you want to load several resources like images or data simultaneously.

When you want to wait for many asynchronous tasks to complete before continuing.

Syntax

Node.js

Promise.all([promise1, promise2, promise3])
  .then(results => {
    // use results array here
  })
  .catch(error => {
    // handle any error from promises
  });

Promise.all takes an array of promises and returns a new promise.

The new promise resolves when all promises succeed, or rejects if any promise fails.

Examples

This example runs two promises that immediately resolve and logs [1, 2].

Node.js

const p1 = Promise.resolve(1);
const p2 = Promise.resolve(2);
Promise.all([p1, p2]).then(results => console.log(results));

This waits for both promises, including one delayed by 100ms, then logs ['a', 'b'].

Node.js

const p1 = Promise.resolve('a');
const p2 = new Promise((res) => setTimeout(() => res('b'), 100));
Promise.all([p1, p2]).then(results => console.log(results));

If any promise rejects, Promise.all rejects immediately with that error.

Node.js

const p1 = Promise.resolve('ok');
const p2 = Promise.reject('error');
Promise.all([p1, p2])
  .then(results => console.log(results))
  .catch(error => console.log('Failed:', error));

Sample Program

This program simulates fetching three pieces of data in parallel with different delays. Promise.all waits for all to finish and then logs the results together.

Node.js

import { setTimeout } from 'node:timers/promises';

async function fetchData(id) {
  await setTimeout(100 * id); // simulate delay
  return `data${id}`;
}

async function run() {
  const promises = [fetchData(1), fetchData(2), fetchData(3)];
  try {
    const results = await Promise.all(promises);
    console.log('All data:', results);
  } catch (error) {
    console.log('Error:', error);
  }
}

run();

Important Notes

Promise.all preserves the order of results matching the order of input promises.

If any promise rejects, Promise.all stops waiting and rejects immediately.

Use Promise.all when tasks are independent and can run at the same time.

Summary

Promise.all runs many promises at once and waits for all to finish.

It returns results in the same order as the promises you gave it.

If one promise fails, the whole Promise.all fails right away.

Practice

1. What does Promise.all do in Node.js?

easy

A. Runs promises one after another in sequence

B. Runs only the first promise and ignores others

C. Runs multiple promises in parallel and waits for all to complete

D. Runs promises but returns results in random order

Solution

1. Step 1: Understand Promise.all behavior

   Promise.all runs all promises at the same time (in parallel) and waits until all finish.

2. Step 2: Check result order and completion

   It returns results in the order of the promises given, not random or sequentially one by one.

3. Final Answer:

   Runs multiple promises in parallel and waits for all to complete -> Option C

4. Quick Check:

   Promise.all = parallel run + wait all [OK]

Hint: Promise.all runs all promises together, not one by one [OK]

Common Mistakes:

• Thinking Promise.all runs promises sequentially
• Believing Promise.all returns results in random order
• Assuming Promise.all ignores failed promises

2. Which of the following is the correct syntax to use Promise.all with an array of promises named promises?

easy

A. Promise.all(promises).then(results => console.log(results));

B. Promise.all.then(promises).catch(error => console.log(error));

C. Promise.all(promises).catch(results => console.log(results));

D. Promise.all(promises).finally(results => console.log(results));

Solution

1. Step 1: Check Promise.all usage

   Promise.all is called as a function with an array of promises as argument.

2. Step 2: Verify chaining with then()

   To get results, use .then() after Promise.all to handle resolved values.

3. Final Answer:

   Promise.all(promises).then(results => console.log(results)); -> Option A

4. Quick Check:

   Correct syntax = Promise.all(array).then() [OK]

Hint: Use Promise.all(array).then() to get results [OK]

Common Mistakes:

• Using Promise.all.then(promises) instead of Promise.all(promises).then()
• Using catch() to handle results instead of errors
• Using finally() to get results instead of then()

3. What will be logged by this code?

const p1 = Promise.resolve(1);
const p2 = Promise.resolve(2);
const p3 = Promise.resolve(3);

Promise.all([p1, p2, p3]).then(results => console.log(results));

medium

A. Error: Promise rejected

B. [1, 2, 3]

C. [undefined, undefined, undefined]

D. [3, 2, 1]

Solution

1. Step 1: Understand resolved promises

   p1, p2, p3 are promises resolved immediately with values 1, 2, and 3.

2. Step 2: Promise.all returns array of results in input order

   Promise.all waits for all to resolve and returns results in the same order as input array.

3. Final Answer:

   [1, 2, 3] -> Option B

4. Quick Check:

   Promise.all results order = input order [OK]

Hint: Promise.all returns results in same order as input promises [OK]

Common Mistakes:

• Assuming results order depends on resolution speed
• Expecting reversed or random order
• Thinking Promise.all returns undefined values

4. Identify the error in this code snippet:

const p1 = Promise.resolve('A');
const p2 = Promise.reject('Error');

Promise.all([p1, p2])
  .then(results => console.log('Results:', results))
  .catch(error => console.log('Caught:', error));

medium

A. The catch block will run with 'Error' because p2 rejects

B. Promise.all will never reject even if one promise fails

C. The then block will run with partial results

D. Syntax error: Promise.reject cannot be used inside Promise.all

Solution

1. Step 1: Check behavior of Promise.all with rejected promises

   If any promise rejects, Promise.all immediately rejects with that error.

2. Step 2: Analyze catch and then blocks

   Since p2 rejects, the catch block runs with the error message 'Error'. The then block does not run.

3. Final Answer:

   The catch block will run with 'Error' because p2 rejects -> Option A

4. Quick Check:

   Promise.all rejects if any promise rejects [OK]

Hint: If one promise rejects, Promise.all rejects immediately [OK]

Common Mistakes:

• Thinking Promise.all ignores rejected promises
• Expecting then block to run with partial results
• Believing Promise.reject causes syntax error here

5. You want to fetch data from three APIs in parallel and process all results only if all succeed. Which code correctly uses Promise.all to achieve this?

const fetch1 = () => fetch('https://api1.example.com/data').then(res => res.json());
const fetch2 = () => fetch('https://api2.example.com/data').then(res => res.json());
const fetch3 = () => fetch('https://api3.example.com/data').then(res => res.json());

// Which code snippet correctly waits for all fetches and handles errors?

hard

A. Promise.all([fetch1(), fetch2(), fetch3()])
.then(results => console.error('All data:', results))
.catch(error => console.log('Fetch failed:', error));

B. Promise.all([fetch1, fetch2, fetch3])
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

C. Promise.all(fetch1, fetch2, fetch3)
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

D. Promise.all([fetch1(), fetch2(), fetch3()])
.then(results => console.log('All data:', results))
.catch(error => console.error('Fetch failed:', error));

Solution

1. Step 1: Check how to call fetch functions

   fetch1, fetch2, fetch3 are functions returning promises, so call them with () to get promises.

2. Step 2: Verify Promise.all usage and error handling

   Pass an array of promises to Promise.all, then use .then() to handle results and .catch() for errors.

3. Final Answer:

   Promise.all([fetch1(), fetch2(), fetch3()]) .then(results => console.log('All data:', results)) .catch(error => console.error('Fetch failed:', error)); -> Option D

4. Quick Check:

   Call functions to get promises, pass array to Promise.all [OK]

Hint: Call functions to get promises before Promise.all [OK]

Common Mistakes:

• Passing functions instead of calling them
• Passing multiple arguments instead of an array
• Swapping console.log and console.error in then/catch