Promise.all and Promise.race

Learn Promise.all for parallel promises and Promise.race.

Parallel work with Promise.all

Promise.all takes an array of promises and returns one promise that fulfils when all of them fulfil. The results arrive in the same order as the input array. If any promise in the array rejects, Promise.all rejects immediately and does not wait for the rest.

Promise.all([loadUser(), loadPosts()])
  .then(results => console.log(results));

This method is ideal for running independent jobs in parallel. Use it when all of them must succeed.

Racing with Promise.race

Promise.race also takes an array, but it settles with the first promise that settles. If the first one fulfils, the result fulfils; if it rejects, the result rejects. The rest are ignored.

Promise.race([fetchWithTimeout(), slowFetch()])
  .then(result => console.log(result));

This method is handy for implementing a timeout pattern. The fastest response wins, so the user does not wait.

Exercise Parallel loading with Promise.all

Two functions called loadProfile and loadStats each return a promise. Run both in parallel with Promise.all. In then receive the results array as the results parameter and log it with console.log. Add catch at the end of the chain and log the error.

Show example solution

Promise.all([loadProfile(), loadStats()])
  .then(results => console.log(results))
  .catch(error => console.log(error));

Test yourself

When does Promise.all reject?
- Never
- When any promise in the array rejects
- When all promises fulfil
- Only when the first promise rejects
Show answer

When any promise in the array rejects — Correct! Promise.all rejects as soon as any promise in the array rejects.

Promise.all call

Promise.____([loadUser(), loadPosts()])
  .then(results => console.log(results));

Show answer

Promise.all([loadUser(), loadPosts()])
  .then(results => console.log(results));

Promise.all and Promise.race

Promise.all

Result when all fulfil
Promise.race

Returns the first to settle
parallel

Run independent jobs at once

Next lesson: Summary - Promises

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