Promise API Static Methods

Promise API Static Methods

What it does: Built-in methods for handling multiple promises at once - like running them in parallel or racing them.

Promise.all() - “All or Nothing”:

Waits for ALL promises to succeed:

javascript

Promise.all([
  fetch('/api/user'),
  fetch('/api/posts'),
  fetch('/api/comments')
]).then(results => {
  // gets array: [userResponse, postsResponse, commentsResponse]
  console.log("All requests done!");
}).catch(error => {
  // if ANY promise fails, this runs immediately
  console.log("Something failed:", error);
});

Key Rule: One failure = whole thing fails immediately.

Promise.allSettled() - “Wait for Everyone”:

Waits for ALL promises, success or failure:

javascript

Promise.allSettled([
  fetch('/api/user'),
  fetch('/bad-url'),      // this will fail
  fetch('/api/posts')
]).then(results => {
  results.forEach(result => {
    if (result.status === 'fulfilled') {
      console.log("Success:", result.value);
    } else {
      console.log("Failed:", result.reason);
    }
  });
});
// Results: [{status: 'fulfilled', value: ...}, {status: 'rejected', reason: ...}, ...]

Promise.race() - “First One Wins”:

Takes result from fastest promise (success OR failure):

javascript

Promise.race([
  fetch('/api/fast'),       // resolves in 100ms
  fetch('/api/slow'),       // resolves in 2000ms
  fetch('/api/timeout')     // rejects in 500ms
]).then(result => {
  console.log("Winner:", result); // gets result from /api/fast
});

Promise.any() - “First Success Wins”:

Takes first SUCCESS, ignores failures:

javascript

Promise.any([
  fetch('/api/broken'),     // fails fast
  fetch('/api/slow'),       // succeeds slow
  fetch('/api/fast')        // succeeds fast
]).then(result => {
  console.log("First success:", result); // gets /api/fast or /api/slow
});

Quick Creation Methods:

javascript

// Create resolved promise
Promise.resolve("data")
  .then(data => console.log(data)); // "data"
 
// Create rejected promise  
Promise.reject(new Error("failed"))
  .catch(error => console.log(error)); // Error: failed

Interview Gotchas:

javascript

// 1. Promise.all preserves order
Promise.all([
  new Promise(resolve => setTimeout(() => resolve("slow"), 1000)),
  new Promise(resolve => setTimeout(() => resolve("fast"), 100))
]).then(results => {
  console.log(results); // ["slow", "fast"] - original order!
});
 
// 2. Non-promises get passed through
Promise.all([
  Promise.resolve(1),
  2,                    // not a promise
  Promise.resolve(3)
]).then(results => {
  console.log(results); // [1, 2, 3]
});
 
// 3. Empty array resolves immediately
Promise.all([]).then(() => console.log("Done!")); // runs immediately

Bottom line: Use Promise.all() when you need all results, Promise.allSettled() when you want to try everything, Promise.race() for fastest response, and Promise.any() for first success.