Understanding Promise.any() (with Custom Implementation)

What is Promise.any()?

Promise.any() is used to run multiple promises in parallel and return the first successfully resolved promise.

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Behavior:

• If any one promise resolves, it immediately returns a resolved promise with that value.

• If all promises reject, it returns a rejected promise with an AggregateError containing all errors.

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Example

const p1 = Promise.reject("Error 1");
const p2 = Promise.resolve(20);
const p3 = Promise.resolve(30);

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

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Rejection Case

const p1 = Promise.reject("Error 1");
const p2 = Promise.reject("Error 2");

Promise.any([p1, p2])
  .then(result => {
    console.log(result);
  })
  .catch(err => {
    console.error(err); 
    // AggregateError: All promises were rejected
    // err.errors => ["Error 1", "Error 2"]
  });

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Key Concept

Promise.any() follows success-first behavior — one success is enough.

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Custom Implementation of Promise.any()

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

Code :

function promiseAny(promises) {
    return new Promise((resolve, reject) => {
        let errors = [];
        let rejectedCount = 0;

        if (promises.length === 0) {
            reject(new Error("Empty iterable"));
        }

        promises.forEach((element, index) => {
            Promise.resolve(element)
                .then((data) => {
                    resolve(data); // first success wins
                })
                .catch((err) => {
                    errors[index] = err;
                    rejectedCount++;

                    if (rejectedCount === promises.length) {
                        reject(new AggregateError(errors, "All promises were rejected"));
                    }
                });
        });
    });
}

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🔗Before You Go Further

If you haven’t read about Promise.all(), check it out here:

Understanding Promise.all (with Custom Implementation)
https://shubhamsinghbundela.hashnode.dev/understanding-promise-all-with-custom-implementation