Async JavaScript explained: promises, fetch and async await

Asynchronous code is where many JavaScript developers lose confidence. Callbacks that run in an unexpected order, promises that never resolve, errors that vanish without a trace, and race conditions when users click faster than the network responds. These problems are not mysterious once you understand a few core ideas.

This free course explains asynchronous JavaScript from the ground up. We start with the event loop in plain words, then build up to promises, async functions, fetch, cancellation, retries and running tasks with limited concurrency.

What you will learn

• How the event loop, task queue and microtask queue decide what runs when
• Creating and consuming promises, and chaining them correctly
• async and await, including error handling and parallel work
• Using fetch properly: status codes, JSON, timeouts and cancellation
• Handling race conditions in user interfaces
• Combinators: Promise.all, allSettled, race and any
• Async iteration and streams at an introductory level

How it works

Each lesson has small examples you can run in the browser console or with Node.js. Several lessons show the same task written three ways, with callbacks, promise chains and async await, so you can read older code as well as write modern code.

You should already know JavaScript basics: functions, arrays, objects and arrow functions.