Quiz Questions

What are Promises and how do they work?

Topics
JAVASCRIPT
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TL;DR

Promises in JavaScript are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. They have three states: pending, fulfilled, and rejected. You can handle the results of a promise using the .then() method for success and the .catch() method for errors.

let promise = new Promise((resolve, reject) => {
// asynchronous operation
if (/* operation successful */) {
resolve('Success!');
} else {
reject('Error!');
}
});
promise.then(result => {
console.log(result); // 'Success!'
}).catch(error => {
console.error(error); // 'Error!'
});

What are Promises and how do they work?

Definition

Promises are a way to handle asynchronous operations in JavaScript. They provide a cleaner, more readable way to handle asynchronous code compared to traditional callback functions.

States of a Promise

A promise can be in one of three states:

  • Pending: The initial state, neither fulfilled nor rejected.
  • Fulfilled: The operation completed successfully.
  • Rejected: The operation failed.

Creating a Promise

You create a promise using the Promise constructor, which takes a function with two arguments: resolve and reject. These are callbacks that you call to change the state of the promise.

let promise = new Promise((resolve, reject) => {
// asynchronous operation
if (/* operation successful */) {
resolve('Success!');
} else {
reject('Error!');
}
});

Handling a Promise

To handle the result of a promise, you use the .then() method for a successful outcome and the .catch() method for an error.

promise
.then((result) => {
console.log(result); // 'Success!'
})
.catch((error) => {
console.error(error); // 'Error!'
});

Chaining Promises

Promises can be chained to handle multiple asynchronous operations in sequence. Each .then() returns a new promise, allowing for further chaining.

promise
.then((result) => {
console.log(result);
return anotherPromise;
})
.then((anotherResult) => {
console.log(anotherResult);
})
.catch((error) => {
console.error(error);
});

Combining Promises

You can use Promise.all() to run multiple promises in parallel and wait for all of them to complete.

let promise1 = Promise.resolve('First');
let promise2 = Promise.resolve('Second');
Promise.all([promise1, promise2]).then((results) => {
console.log(results); // ['First', 'Second']
});

Further reading

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