Q506: How does Promise.allSettled() differ from Promise.all()? - JavaScript Internals (521 Qs)

Promise.allSettled() and Promise.all() both handle multiple promises, but they behave differently when promises are rejected.

Promise.all() - fails fast:

javascript

 const promises = [
   Promise.resolve(1),
   Promise.reject('Error'),
   Promise.resolve(3)
 ];

 Promise.all(promises)
   .then(results => console.log(results))
   .catch(error => console.log(error)); // 'Error'
 // Stops at first rejection

Promise.allSettled() - waits for all:

javascript

 Promise.allSettled(promises)
   .then(results => console.log(results));
 /*
 [
   { status: 'fulfilled', value: 1 },
   { status: 'rejected', reason: 'Error' },
   { status: 'fulfilled', value: 3 }
 ]
 */

Practical example - multiple API calls:

javascript

 async function fetchUserData(userId) {
   const endpoints = [
     fetch(`/api/users/${userId}`),
     fetch(`/api/users/${userId}/posts`),
     fetch(`/api/users/${userId}/comments`)
   ];

   const results = await Promise.allSettled(endpoints);

   return {
     profile: results[0].status === 'fulfilled' 
       ? await results[0].value.json() 
       : null,
     posts: results[1].status === 'fulfilled' 
       ? await results[1].value.json() 
       : [],
     comments: results[2].status === 'fulfilled' 
       ? await results[2].value.json() 
       : []
   };
 }

Filtering settled results:

javascript

 const results = await Promise.allSettled(promises);

 const successful = results
   .filter(r => r.status === 'fulfilled')
   .map(r => r.value);

 const failed = results
   .filter(r => r.status === 'rejected')
   .map(r => r.reason);

 console.log(`${successful.length} succeeded, ${failed.length} failed`);

When to use each:

Use Promise.all() when:	Use Promise.allSettled() when:
All promises must succeed	You need all results regardless of status
Failure should stop execution	You want to handle each result independently
You want to fail fast	You need a complete report