Q82- Write a custom Promise.all() implementation.

Promise.all() takes an array of promises and returns a single promise that:

1. Resolves when all promises succeed → returns an array of results.
2. Rejects immediately if any promise fails → returns the first error.

We can implement a custom version using basic Promise logic.

---

1️⃣ Custom Implementation

function myPromiseAll(promises) {
  return new Promise((resolve, reject) => {
    if (!Array.isArray(promises)) {
      return reject(new TypeError("Input must be an array"));
    }

    const results = [];
    let completed = 0;

    promises.forEach((p, index) => {
      // Ensure non-promises are wrapped in Promise.resolve
      Promise.resolve(p)
        .then((value) => {
          results[index] = value;
          completed++;

          // If all promises are done, resolve with results
          if (completed === promises.length) {
            resolve(results);
          }
        })
        .catch((err) => {
          reject(err); // Reject immediately if any promise fails
        });
    });

    // Handle empty array
    if (promises.length === 0) {
      resolve([]);
    }
  });
}

---

2️⃣ Example Usage

const p1 = Promise.resolve(1);
const p2 = new Promise((res) => setTimeout(() => res(2), 500));
const p3 = Promise.resolve(3);

myPromiseAll([p1, p2, p3])
  .then((results) => console.log("Results:", results))
  .catch((err) => console.error("Error:", err));

✅ Output after 500ms:

Results: [1, 2, 3]

---

3️⃣ Key Points

• Promise.resolve(p) ensures that even non-promise values are handled.
• Results are stored in the original order using the index.
• Rejects immediately if any promise fails, just like native Promise.all().
• Handles empty arrays by resolving immediately with [].

---

In short:

myPromiseAll mimics Promise.all() by tracking completions, storing results in order, and rejecting immediately if any promise fails.