.all()

The .all() method returns a new Promise object that can be accessed as an array of resolved values of fulfilled promises. It takes an iterable object, such as an Array, that contains one or more Promise objects. This is ideal when working with promises that depend on one another’s completion.

Syntax

Promise.all(iterableObject);

The iterableObject is usually an array of Promise objects. If the array is empty, a Promise object that resolves into an empty array will be returned.

Example

Working with two promises, promiseA and promiseB:

const promiseA = new Promise((resolve, reject) => {
  resolve(23);
});

const promiseB = new Promise((resolve, reject) => {
  if (12 * 12 === 144) {
    resolve(144);
  } else {
    reject({
      errorType: 'TypeError',
      message: `Unexpected type - expected ${typeof (12 * 12)}.`,
    });
  }
});

Promise.all([promiseA, promiseB])
  .then((values) => {
    console.log(`Results from Promise.all(): [${values}]`);
  })
  .catch((err) => {
    console.log(`Promise.all Failed! \n${err.errorType}: ${err.message}`);
  })
  .finally(() => {
    console.log('Operations for Promise.all() have finished.');
  });

The output would be:

Results from Promise.all(): [23,144]
Operations for Promise.all() have finished.

Codebyte Example

The following example demonstrates that the promise returned from Promise.all resolves only if all promises (passed as an array) resolve. The resolved value is an array containing the values of each resolved promise.

If at least one promise was rejected, Promise.all rejects with the value of the first rejected promise it encounters.

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