Understanding Promise.all (with Custom Implementation)

What is Promise.all()?

Promise.all() is used to run multiple promises in parallel and wait until all of them complete.

Behavior:

• If all promises resolve, it returns a resolved promise with an array of results.

• If any one promise rejects, it immediately returns a rejected promise with that error.

• It fails fast — meaning it doesn’t wait for other promises once one fails.

Example

const p1 = Promise.resolve(10);
const p2 = Promise.resolve(20);
const p3 = Promise.resolve(30);

Promise.all([p1, p2, p3])
  .then(result => {
    console.log(result); // [10, 20, 30]
  })
  .catch(err => {
    console.error(err);
  });

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Rejection Case

const p1 = Promise.resolve(10);
const p2 = Promise.reject("Error in p2");
const p3 = Promise.resolve(30);

Promise.all([p1, p2, p3])
  .then(result => {
    console.log(result);
  })
  .catch(err => {
    console.error(err); // "Error in p2"
  });

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Key Concept

Promise.all() follows fail-fast behavior — one failure stops everything.

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Custom Implementation of Promise.all()

Let’s now understand how Promise.all() works internally by implementing it ourselves.

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Code

function promiseAll(promises) {
    return new Promise((resolve, reject) => {
        let arr = [];
        let completed = 0;

        if (promises.length === 0) {
            resolve([]);
        }

        promises.forEach((element, index) => {
            Promise.resolve(element)
                .then((data) => {
                    arr[index] = data;
                    completed++;

                    if (completed === promises.length) {
                        resolve(arr);
                    }
                })
                .catch(reject);
        });
    });
}