CodeBlanks Logo Icon CodeBlanks
Login Register

Promise.all

Learn parallel promises with Promise.all.

What is Promise.all?

To start several async jobs in parallel and wait for all of them you use Promise.all. It takes an array of Promises and, when all are fulfilled, returns an array of their results. Since the jobs run in parallel, the total time is roughly that of the slowest one.

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

Important: if any Promise in the array rejects, Promise.all immediately rejects with that error; even if the others finish, no result array is produced. When all Promises succeed you get the array; if even one rejects, you get an error.

Result order

The array returned by Promise.all keeps the order of the input Promises. Even if the second Promise finishes before the first, it still appears in the second position of the result array. This is handy when you need to process parallel results in a fixed order.

Promise.all([p1, p2])
  .then(values => console.log(values[0], values[1]));

Here p1 and p2 run in parallel; once both finish, the result array arrives as [result1, result2]. Speed differences do not matter; the order matches the input.

Practice Your turn: parallel with Promise.all

Create two Promises: const p1 = Promise.resolve(10) and const p2 = Promise.resolve(20). Call Promise.all([p1, p2]) and log the sum with .then(values => console.log(values[0] + values[1])).

Starter code

index.js
// Create p1 and p2 Promises
// Combine with Promise.all, log the sum in .then

Sample solution

index.js
const p1 = Promise.resolve(10);
const p2 = Promise.resolve(20);
Promise.all([p1, p2]).then(values => console.log(values[0] + values[1]));

Expected output

30

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 When does Promise.all reject?

  • When the first Promise resolves
  • When any Promise in the array rejects Correct answer
  • When all Promises reject
  • When all Promises fulfill

Explanation: The first rejected Promise rejects the whole chain.

2 Fill in the blank: Promise.all

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

Explanation: Promise.all returns all results as an array.

3 Promise.all behaviours

Term Meaning
Promise.all resolves when all done
parallel runs concurrently
one rejects rejects the whole

All lessons in Beginner

1 Node.js basics

  1. What is Node.js?
  2. Your first program
  3. ES modules
  4. package.json
  5. CommonJS vs ESM
  6. Summary - Node.js basics

2 Variables

  1. let and const
  2. var and scope
  3. Type inference
  4. Destructuring
  5. Spread and rest
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Template literals
  3. Number and Math
  4. Symbol
  5. typeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. process.argv
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Nullish coalescing
  6. Summary - Operators

6 Strings

  1. String methods
  2. String search
  3. substring and slice
  4. replace and split
  5. padStart and trim
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Ternary
  5. Optional chaining
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. for...of
  4. for...in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. push and pop
  4. shift and unshift
  5. splice and slice
  6. Summary - Arrays

11 Array methods

  1. map
  2. filter
  3. reduce
  4. find and some
  5. forEach
  6. Summary - Array methods

12 Objects

  1. Object literal
  2. Property access
  3. Computed properties
  4. Object methods
  5. Object spread
  6. Summary - Objects

13 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Default parameters
  5. Rest parameters
  6. Summary - Functions

14 Callbacks

  1. What are callbacks?
  2. Higher-order functions
  3. forEach with callbacks
  4. Error-first callbacks
  5. Callback hell
  6. Summary - Callbacks

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Promises

  1. What are promises?
  2. then and catch
  3. Promise.all
  4. Promise.race
  5. finally
  6. Summary - Promises

17 async/await

  1. async function
  2. await
  3. try/catch
  4. await with Promise.all
  5. Sequential await
  6. Summary - async/await

18 fs module

  1. fs.promises
  2. readFile
  3. writeFile
  4. appendFile
  5. existsSync
  6. Summary - fs module

19 path module

  1. path.join
  2. path.resolve
  3. path.parse
  4. path.dirname and basename
  5. path.extname
  6. Summary - path module

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. fs persistence
  5. Putting it together