28. Promise.all Polyfill

Implement your own version of Promise.all method. The Promise.all method takes an iterable of promises and returns a single promise that resolves when all of the input promises have resolved, or rejects with the reason of the first promise that rejects.

1. Basic Functionality

• Accept an iterable of promises (array, Set, etc.)
• Return a Promise that resolves with an array of results
• Preserve the order of results (results array matches input order)
• Resolve only when all promises resolve

2. Error Handling

• Reject immediately when any promise rejects (fail-fast behavior)
• Reject with the reason of the first rejected promise
• Prevent multiple rejections

3. Edge Cases

• Handle empty iterable (should resolve with empty array)
• Handle non-promise values (should wrap them with Promise.resolve)
• Handle already settled promises
• Support any iterable, not just arrays

Examples

// All promises resolve
promiseAll([
  Promise.resolve(1),
  Promise.resolve(2),
  Promise.resolve(3)
]).then(console.log); // [1, 2, 3]

// One promise rejects
promiseAll([
  Promise.resolve(1),
  Promise.reject('error'),
  Promise.resolve(3)
]).catch(console.log); // 'error'

// Mixed values
promiseAll([1, Promise.resolve(2), 3])
  .then(console.log); // [1, 2, 3]

// Empty array
promiseAll([])
  .then(console.log); // []