What are Promise combinators and when do you use each?

• Promise.all() — waits for ALL to resolve. Rejects immediately if ANY rejects. Use when all must succeed.
• Promise.allSettled() — waits for ALL to settle (resolve OR reject). Never rejects itself. Use when you need all results regardless of failure.
• Promise.race() — settles with the FIRST settled promise (resolve or reject). Use for timeout patterns.
• Promise.any() — resolves with the FIRST resolved promise. Rejects only if ALL reject. Use for fallback/redundancy.

const fast = fetch('/fast');   // 100ms
const slow = fetch('/slow');   // 500ms
const bad  = fetch('/broken'); // 200ms, rejects

await Promise.all([fast, slow]);    // ✅ 500ms (waits for both)
await Promise.all([fast, bad]);     // ❌ rejects at 200ms

const results = await Promise.allSettled([fast, slow, bad]);
// [{status:'fulfilled', value:...}, ..., {status:'rejected', reason:...}]

await Promise.race([fast, slow]);   // resolves at 100ms with fast result

await Promise.any([bad, fast]);     // resolves at 100ms (ignores bad)

// Timeout pattern with race:
const withTimeout = (p, ms) => Promise.race([
  p,
  new Promise((_, r) => setTimeout(() => r(new Error('Timeout')), ms))
]);