Asynchronous JavaScript

What you will learn

• The event loop
• Timers
• Callbacks
• Promises
• async and await
• Promise.all, allSettled, race and any
• fetch and HTTP
• AbortController
• Retries with backoff

Lessons

1. 1

   The Event Loop and Timers

   See how one thread runs your code to the end before any callback, schedule and cancel timers, predict task and microtask order, and keep long work from freezing the page.

   14 exercises

2. 2

   Callbacks

   Deliver results from asynchronous work through callbacks, follow the error-first convention, call a callback exactly once, and see why nesting and handing over control led to promises.

   12 exercises

3. 3

   Promises

   Create promises and promisify callbacks, chain then and return from every step, recover with catch and clean up with finally, and end every chain so no rejection goes unhandled.

   12 exercises

4. 4

   Promise Patterns and async/await

   Write async functions with await and try/catch, run independent work in parallel with Promise.all and allSettled, set deadlines with race, take the first success with any, and choose sequential or parallel awaits on purpose.

   12 exercises

5. 5

   fetch and HTTP

   Call HTTP APIs with fetch: check response.ok because fetch does not reject on a 404, parse and send JSON, use GET, POST, PUT and DELETE, build query strings, read a CORS error, recognize XMLHttpRequest and cancel with AbortController.

   12 exercises

6. 6

   Putting It Together

   Build the async pieces real apps need: a weather client that retries with backoff, a loader that follows every page, a concurrency limit and a cached fetch that shares requests.

   9 exercises

How you practice

You practice in the browser and every exercise gives you feedback right away. This course uses these formats:

• Code exercise: 25
• Fix the bug: 12
• Predict the output: 9
• Fill in the blank: 8
• Multiple choice: 6
• Type the answer: 4
• Match the pairs: 4
• Reorder lines: 3