Week 10 - Asynchronous programming

Synchronous vs Asynchronous Code

Callbacks

Promises

Asynchronous File I/O

Event Loop

Fetch API

Async/Await and Promise.all()

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Practice

Assignment

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Week 10 Overview

This week, you’ll step beyond simple top-to-bottom programs and learn how JavaScript deals with work that cannot finish immediately. Real applications need to talk to servers, read and write files, wait for timers, and react to user input, all without freezing the interface. Asynchronous programming is the set of tools and ideas that makes this possible, and it is a core skill for any JavaScript developer.

Instead of focusing only on syntax, this module is about building a strong mental model. You’ll see how JavaScript schedules work over time, how seemingly “out of order” logs still follow clear rules, and how different approaches to asynchronous code affect the structure and readability of your programs. By the end of the week, you should feel more confident understanding and debugging asynchronous behaviour in real projects, and ready to apply these ideas in the rest of the Core program and beyond.

Learning goals

• Explain the difference between synchronous and asynchronous code, and what “blocking” means in JavaScript.
• Use callbacks and timer functions (setTimeout, setInterval) to schedule work and avoid blocking the main thread.
• Recognize “callback hell” and explain why it makes code hard to read and maintain.
• Create and use Promises, and work with their states (pending, fulfilled, rejected).
• Handle asynchronous results and errors using then, catch, and finally, including basic promise chains.
• Perform asynchronous file I/O in Node.js using fs/promises, and handle errors from file operations.
• Describe, at a conceptual level, how the event loop, call stack, task queue, and microtask queue work together.
• Predict the order in which logs will appear when mixing synchronous code, timers, and promise callbacks.
• Use the Fetch API in Node.js to make GET and POST requests, parse JSON responses, and handle HTTP errors correctly.
• Rewrite promise-based code using async / await and try / catch, and explain how async functions relate to promises.
• Decide when asynchronous operations should be sequential and when they can run in parallel.
• Use Promise.all() (with or without async / await) to run multiple asynchronous operations in parallel and work with all results together.

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