A Brief Guide to Promises in JavaScript

A Promise in JavaScript is an object which returns a result after an asynchronous operation has finished. The result of promise can either be a success or a failure.

The Promise has 3 states,

Pending — The Promise has no result yet.
Fulfilled — The Promise has resolved.
Rejected — The Promise has rejected.

let promise = new Promise((resolve, reject) => {
    const number = Math.round(Math.random() * 10);
    if(number <= 5) {
        resolve('Success');
    } else {
        reject(new Error('Failed'));
    }
});

To create a Promise we use the Promise() constructor with the new keyword. The Promise() constructor accepts a callback function called “executer” which accepts two arguments i.e. resolve and reject.

We need to call the resolve() function when the executer function obtains the result and call the reject() function when there is an error.

Now to consume the Promise we use the Promise methods like then(), catch() and finally().

Promise.then()

The then() method can accept two callback functions, first one executes when the Promise is resolved and second one when Promise is rejected.

function getPromise(isTrue) {
    return new Promise((resolve, reject) => {
        if(isTrue) {
            resolve('Success');
        } else {
            reject(new Error('Failed'));
        }
    });
}
getPromise(true).then(
    response => console.log('Promise is resolved with result = ' + response),
    error => console.log('Promise is rejected with error = ' + error.message)
);
// Promise is resolved with result = Success
getPromise(false).then(
    response => console.log('Promise is resolved with result = ' + response),
    error => console.log('Promise is rejected with error = ' + error.message)
);
// Promise is rejected with error = Failed

If we want to handle success and failure cases separately then we can use then() only for success and catch() for failure.

Promise.catch()

The catch() method takes a callback function which executes when the Promise is rejected.

function getPromise(isTrue) {
    return new Promise((resolve, reject) => {
        if(isTrue) {
            resolve('Success');
        } else {
            reject(new Error('Failed'));
        }
    });
}
getPromise(false)
.then(response => console.log('Promise is resolved with result = ' + response))
.catch(error => console.log('Promise is rejected with error = ' + error.message))
// Promise is rejected with error = Failed

If we are handling errors both in then() and catch(), then only the error handler inside then() executes in case of any error and not the handler inside catch().

let promise = new Promise((resolve, reject) => {
    reject(new Error('An error occurred'));
});
promise.then(null, () => console.log('Error caught inside then'))
.catch(() => console.log('Error caught inside catch'))
// Error caught inside then

If further any error occurred in error handler of then(), then it will get caught in catch().

let promise = new Promise((resolve, reject) => {
    reject(new Error('Error occurred in Promise'));
});
promise.then(null, 
    err => {
        console.log('Error caught inside then, message: ' + err.message);
        throw new Error('Error occurred in then');
    })
.catch(err => {
    console.log('Error caught inside catch, message: ' + err.message);
});
// Error caught inside then, message: Error occurred in Promise
// Error caught inside catch, message: Error occurred in then

Promise.finally()

The finally() method takes a callback function which executes after the Promise is either resolved or rejected.

On Success

let promise = new Promise((resolve, reject) => {
    resolve('Success');
});
promise.then(res => console.log(res))
.catch(err => console.log(err.message))
.finally(() => console.log('Executing Finally'));
// Success
// Executing Finally

On failure

let promise = new Promise((resolve, reject) => {
    reject(new Error('Failed'));
});
promise.then(res => console.log(res))
.catch(err => console.log(err.message))
.finally(() => console.log('Executing Finally'));
// Failed
// Executing Finally

Promises Chaining

We can execute a sequence of asynchronous operations one after other by chaining methods then(), catch() and finally().

takeOrder()
.then(order => makeOrder(order))
.then(order => serveOrder(order))
.then(status => console.log(status))
.catch(err => console.log(err));

Here takeOrder() will return a Promise that will be consumed in first then() method. The makeOrder() will return a Promise which will be consumed in second then() method and serveOrder() will again return a Promise which will be consumed in third then() method. If any error occurred in any of the promises then it will get caught in catch() method.

Promise.all()

The Promise.all() method takes an iterable of promises as an input, and returns a single Promise and it gets resolved when all the promises get resolved or any one of them gets rejected.

function getPromise(delay) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(delay + 100);
        }, delay);
    })
}
Promise.all([getPromise(1000), getPromise(3000), getPromise(2000)])
.then(responses => console.log(responses))
.catch(error => console.log(error));

Promise.race()

The Promise.race() method takes an iterable of promises and returns a single Promise which gets resolved/rejected as soon as any of the Promises resolved or rejected.

let promise1 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('Promise 1');
    }, 1000);
});
let promise2 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('Promise 2');
    }, 500);
});
Promise.race([promise1, promise2])
.then(res => console.log(res)) // Promise 2
.catch(err => console.log(err));