Mastering Asynchrony with Promises in JavaScript

Escape the chains of synchronous code and learn how to handle background tasks gracefully without freezing your application.

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Welcome! JavaScript runs code line-by-line. Let's see what happens with a slow task.

1. Do task A ✅
2. Do task B ✅
3. Do task C ✅

The Problem: Synchronous vs. Asynchronous

JavaScript is single-threaded, meaning it does one thing at a time. If a slow task (like fetching data from a server) runs, it blocks everything else, freezing the user interface. Asynchrony allows these slow tasks to run in the background without blocking the main thread.

The Solution: What is a Promise?

A Promise is a special JavaScript object that acts as a placeholder for a future result. It "promises" to give you a value at some point. This lets your code continue running while the asynchronous operation completes in the background.

The Three States of a Promise

A Promise is always in one of three states: Pending (the initial state; the operation hasn't finished), Fulfilled (the operation completed successfully and has a result), or Rejected (the operation failed and has an error).

Handling Results: .then() and .catch()

We interact with promises using handler methods. .then() is called when a promise is fulfilled, receiving the result. .catch() is called when a promise is rejected, receiving the error. .finally() is called regardless of the outcome, perfect for cleanup tasks.

Practice Zone

Interactive Test 1: Order the Promise Chain

Drag the methods into the correct order to handle a Promise.

Arrastra en el orden correspondiente.

Arrastra las opciones:

.catch()

new Promise()

.then()

Completa el código:

Promise...______

Success Handler______

Error Handler______

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Interactive Test 2: Build a Promise

A Promise executor provides two functions. Fill in their conventional names.

Rellena los huecos en cada casilla.

new Promise(( ,  ) => {
  // Async operation...
});

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Practice Example: Code Your Own Promise

Create a promise that simulates fetching data. It should resolve with the string "Data received!" after 1.5 seconds.

Enunciado:

* Escribe el código a continuación. Los caracteres correctos se mostrarán en verde y los incorrectos en rojo.

new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve("Data received!");
    }, 1500);
});

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Knowledge Check

Which method is specifically used to handle a rejected Promise?

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Promises in the Real World

Promises are the backbone of modern asynchronous JavaScript. Here are the most common ways you'll use them.

1. Fetching API Data

The fetch() API is the modern way to make network requests. It returns a Promise that resolves to the Response object representing the response to your request.

fetch('https://api.example.com/data')
  .then(response => response.json()) // .json() also returns a promise!
  .then(data => console.log(data))
  .catch(error => console.error('Fetch error:', error));

2. Handling Multiple Requests with `Promise.all()`

Sometimes you need to wait for multiple asynchronous operations to complete. Promise.all() takes an array of promises and returns a single new promise that fulfills when all of the input promises have fulfilled.

const promise1 = fetch('/api/users');
const promise2 = fetch('/api/posts');

Promise.all([promise1, promise2])
  .then(responses => {
    // Both requests are complete
    console.log('Got users and posts!');
  });

3. The Elegance of `async/await`

async/await is "syntactic sugar" on top of promises. It lets you write asynchronous code that is easier to read and reason about, especially when dealing with multiple sequential asynchronous steps.

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Something went wrong:', error);
  }
}

Practical Takeaway: Whether you use .then() chains or the async/await syntax, understanding the underlying Promise object is crucial for writing robust, non-blocking JavaScript applications.

Asynchrony Glossary

Asynchrony: The ability to perform long-running tasks without blocking the main execution thread, keeping the application responsive.
Promise: An object representing the eventual completion (or failure) of an asynchronous operation and its resulting value.
resolve(value): A function called within a Promise's executor to mark it as fulfilled, passing along the successful result.
reject(error): A function called within a Promise's executor to mark it as rejected, passing along the reason for failure.
.then(): A method that registers a callback to run when the Promise is fulfilled.
.catch(): A method that registers a callback to run when the Promise is rejected.
async: A keyword that declares a function as asynchronous, ensuring it implicitly returns a Promise.
await: An operator, only usable inside an `async` function, that pauses execution and waits for a Promise to settle before resuming.