Promise in javascript

Promise.all()

The Promise.all() method takes an iterable of promises as an input, and returns a single Promise that resolves to an array of the results of the input promises.

const promise1 = Promise.resolve(3);
const promise2 = 42;
const promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
});

Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values);
});
// expected output: Array [3, 42, "foo"]

Promise.allSettled()

The Promise.allSettled() method returns a promise that resolves after all of the given promises have either fulfilled or rejected, with an array of objects that each describes the outcome of each promise.

const promise1 = Promise.resolve(3);
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 100, 'foo'));
const promises = [promise1, promise2];

Promise.allSettled(promises).
  then((results) => results.forEach((result) => console.log(result.status)));

// expected output:
// "fulfilled"
// "rejected"

Promise.any()

Promise.any() takes an iterable of Promise objects. It returns a single promise that resolves as soon as any of the promises in the iterable fulfills, with the value of the fulfilled promise. If no promises in the iterable fulfill (if all of the given promises are rejected), then the returned promise is rejected with an AggregateError,

const promise1 = Promise.reject(0);
const promise2 = new Promise((resolve) => setTimeout(resolve, 100, 'quick'));
const promise3 = new Promise((resolve) => setTimeout(resolve, 500, 'slow'));

const promises = [promise1, promise2, promise3];

Promise.any(promises).then((value) => console.log(value));

// expected output: "quick"

Promise.prototype.catch()

The catch() method returns a Promise and deals with rejected cases only. It behaves the same as calling Promise.prototype.then(undefined, onRejected) (in fact, calling obj.catch(onRejected) internally calls obj.then(undefined, onRejected)). This means that you have to provide an onRejected function even if you want to fall back to an undefined result value - for example obj.catch(() => {}).

const promise1 = new Promise((resolve, reject) => {
  throw 'Uh-oh!';
});

promise1.catch((error) => {
  console.error(error);
});
// expected output: Uh-oh!

Promise.prototype.finally()

The finally() method returns a Promise. When the promise is settled, i.e either fulfilled or rejected, the specified callback function is executed. This provides a way for code to be run whether the promise was fulfilled successfully or rejected once the Promise has been dealt with.

This helps to avoid duplicating code in both the promise's then() and catch() handlers.

function checkMail() {
  return new Promise((resolve, reject) => {
    if (Math.random() > 0.5) {
      resolve('Mail has arrived');
    } else {
      reject(new Error('Failed to arrive'));
    }
  });
}

checkMail()
  .then((mail) => {
    console.log(mail);
  })
  .catch((err) => {
    console.error(err);
  })
  .finally(() => {
    console.log('Experiment completed');
  });

Promise.race()

The Promise.race() method returns a promise that fulfills or rejects as soon as one of the promises in an iterable fulfills or rejects, with the value or reason from that promise.

const promise1 = new Promise((resolve, reject) => {
  setTimeout(resolve, 500, 'one');
});

const promise2 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'two');
});

Promise.race([promise1, promise2]).then((value) => {
  console.log(value);
  // Both resolve, but promise2 is faster
});
// expected output: "two"

Promise.reject()

The Promise.reject() method returns a Promise object that is rejected with a given reason.

function resolved(result) {
  console.log('Resolved');
}

function rejected(result) {
  console.error(result);
}

Promise.reject(new Error('fail')).then(resolved, rejected);
// expected output: Error: fail

Promise.resolve()

The Promise.resolve() method returns a Promise object that is resolved with a given value. If the value is a promise, that promise is returned; if the value is a thenable (i.e. has a "then" method), the returned promise will "follow" that thenable, adopting its eventual state; otherwise the returned promise will be fulfilled with the value. This function flattens nested layers of promise-like objects (e.g. a promise that resolves to a promise that resolves to something) into a single layer.

Promise.resolve(value);

Promise.prototype.then()

The then() method returns a Promise. It takes up to two arguments: callback functions for the success and failure cases of the Promise.

const promise1 = new Promise((resolve, reject) => {
  resolve('Success!');
});

promise1.then((value) => {
  console.log(value);
  // expected output: "Success!"
});

Wrapping code with promise

return new Promise((resolve, reject) => {
...
resolve("returned val")
})