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Asynchronous JavaScript

What is asynchronous JavaScript?​

Asynchronous (async) JavaScript is JavaScript that starts a task now and finishes it later. While that task is waiting, the program can keep doing other work.

This is important because many operations take time:

  • Fetching data from a server
  • Reading files
  • Waiting for user input
  • Timers and animations

If JavaScript had to wait for these operations to finish before doing anything else, programs would feel frozen and unresponsive.

Why async exists​

JavaScript is single-threaded, meaning it runs your JavaScript code one step at a time. But the environment (the browser or Node.js) can do work in the background, like waiting for a timer or a network response.

Async programming is how you start those background tasks and handle the result when it is ready.

A tiny async example​

This example shows the basic async idea: code continues running while a timer waits.

console.log("Start");

setTimeout(() => {
  console.log("Later");
}, 0);

console.log("End");

Output:

Start
End
Later

Even with a 0 millisecond delay, the timer callback runs after the current synchronous code finishes.

The three common async styles​

JavaScript codebases commonly use three styles for async work:

  1. Callbacks: you pass a function to run later.
  2. Promises: an object that represents a value you will have in the future.
  3. async / await: syntax that makes promise-based code read more like top-to-bottom code.

In modern JavaScript, you will usually write promises and async / await.

What this section covers​

This section teaches modern async JavaScript:

  1. Callbacks: Learn to recognize callback style and understand why promise-based code is easier to manage.
  2. Promises: The core tool for representing future values.
  3. async / await: The most common way to write promise-based code.

Recommended order: start with promises, then learn async / await.

What this section does not cover​

  • A deep dive into the event loop
  • Browser and Node async APIs in detail (fetch, setTimeout, DOM events)
  • Advanced promise compositions and edge cases
  • Generators and async iterators

Focus on mastering promises and async / await first. They cover most real-world async code.