These days you can rarely write a JavaScript program without thinking about asynchronous code. You must understand this deeply.

In this section we’ll talk about Callbacks, Promises, Async/Await.

What is asynchronous code?

Suppose your program needs to perform some kind of action.

This action could take multiple seconds to perform. You don’t want to halt your entire program to wait for its completion, right?

That’s why we need a way to tell JavaScript “do this, and get back to me when you’re done, but in the meantime continue doing some other work”.

We’ll introduce this topic in the same historical order of appearance of the ways we can do it.

JavaScript, since its early days, had callbacks, paired with timers and other browser-provided APIs like event handlers.

Then we got promises and async functions, two relatively recent addition to the language, which completely transformed the way we now write JavaScript.

We’ll first learn callbacks, then we’ll dive into promises and async functions.

Lessons in this unit:

0: ▶︎ Introduction
1: Callbacks
2: Timers
3: Promises
4: Async functions
5: Chaining promises
6: Orchestrating promises
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