Javascript is a single threaded language. This means it has one call stack and one memory heap. As expected, it executes code in order and must finish executing a piece code before moving onto the next. It's synchronous, but at times that can be harmful. For example, if a function takes awhile to execute or has to wait on something, it freezes everything up in the meanwhile.

A good example of this happening is the window alert function.

  alert("Hello World!");

You can't interact with the webpage at all until you hit OK and dismiss the alert. You're stuck.

So how do we get asynchronous code with Javascript then?

Well, we can thank the Javascript engine (V8, Spidermonkey, JavaScriptCore, etc...) for that, which has Web API that handle these tasks in the background. The call stack recognizes functions of the Web API and hands them off to be handled by the browser. Once those tasks are finished by the browser, they return and are pushed onto the stack as a callback.

Open your console and type "window" then press enter. You'll see most everything the Web API has to offer. This includes things like ajax calls, event listeners, the fetch API, and setTimeout. Javascript uses low level programming languages like C++ to perform these behind the scenes.

Let's look at a simple example, run this code your console:

  setTimeout(() => {
  }, 1000)

What did we get back?


Feels odd, right? Well, let's break this down line by line:

"console.log("first")" is on the stack first, so it gets printed. Next, the engine notices setTimeout, which isn't handled by Javascript and pushes it off to the WebAPI to be done asynchronously. The call stack moves on without caring about the code handed off to the Web APIs and "console.log("three")" is printed.

Next, the Javascript engine's event loop kicks in, like a little kid asking "Are we there yet?" on a road trip. It starts firing, waiting for events to be pushed into it. Since the "setTimeout" isn't finished, it returns "undefined", as the default, well because it hasn't been given the value yet. Once the callback finally does hits we get "console.log("second")" printed.

There's a really good site that slows this all down and shows this happening.


I suggest playing around in this sandbox to help solidify your understanding. It helped me get a feel for how asynchronous code can work with Javascript being single threaded.