Javascript Internals: What's Under the Hood?

Written by Priyath Gregory on December 16, 20199 min read

You really don’t need to know a lot to get up and running with Javascript. In fact, a fully-fledged production-ready app could be built without ever understanding the inner workings of the language. This is both a blessing and a curse; The language has many adopters, but only a very few masters.

This realization led me on a quest to try and understand how Javascript really works under the hood. The journey is far from over, but the fog has started to lift. And it all started with the question, “What exactly is Javascript?”.

So what is Javascript? To put it formally, Javascript is a single-threaded, non-blocking, asynchronous programming language.

…

Wait what?

On the surface, this definition (at least for me) was contradictory, confusing, and just lead to more questions. How could a single-threaded program, be non-blocking? How could it also be asynchronous?

These are all very valid questions to ask; But bear with me, they also have answers. Answers that lie deep within the heart of Javascript, its runtime environment.

The Javascript Runtime

The Javascript runtime environment is what makes your code work, it is what allows Javascript to have all the complex terminology in its formal definitions, and from the context of a browser, it is mainly a combination of 4 things:

1. The Javascript engine

2. Web APIs

3. The callback queue

4. The event loop

Let us examine each of these pieces individually.

The Javascript Engine

The Javascript engine is the brain of the runtime environment and its main purpose is to convert your human-readable scripts into machine-readable code.

It also manages a call stack (think of a LIFO stack data structure) to keep track of the execution of your code, and the execution itself is always performed on a single thread, synchronously. Additionally, the engine performs garbage collection, optimizations, and a whole bunch of other things that deserves its own article.

Any engine worth your time will also contain an implementation of ECMAScript, a scripting-language specification created to standardize Javascript. This is what allows the engine to “know” what a while loop is, and what Math.round(x) is supposed to do. If all you need is a bunch of while loops, a few function definitions, and some variables (in other words, if what you need is defined in the ECMAScript specification), the Javascript engine is quite capable of handling your code on its own.

But Javascript is much more than just a few function definitions. It is a powerful language that is capable of doing a lot of things, which brings us to the next piece of the puzzle, the web APIs.

Web APIs

If you check out the source code of a Javascript engine such as V8, you will realize that many functions that you thought were a part of the standard Javascript specification, are simply not there. A case in point is the setTimeout function. I don't know about you, but for me setTimeout is standard Javascript. But if it's not there within the engine, where is it?

Enter Web APIs. Web APIs are key browser-specific implementations that extend the Javascript language. For example, if you open the Google Chrome console and type in something like:

function main(){
  setTimeout(()=>console.log('Hello World!'), 5000);
};
main();

“Hello World” will be printed to the console after 5 seconds as expected. This code works because the Web APIs of Google Chrome have their own implementation of setTimeout embedded into its Javascript engine, V8. Similarly, the DOM is also not a part of the Javascript engine. Anything DOM-related: be it, event listeners, accessors, or manipulators, are all defined in.. you guessed it, web APIs!

A very logical question to ask at this point is if the Javascript engine does not know of these functions, then how would it handle a “web API” function call in my script?

The answer lies in how the browser embeds web APIs to its Javascript engine.

When the Javascript engine is looking to resolve a symbol, it will start in the local scope and go up a chain of scopes. At the very end of the chain is the global scope. As part of initializing the Javascript engine, any host environment can add its own APIs to this global scope, thus exposing the function and its corresponding handler to the Javascript engine.

To better understand this, let's go back to our example:

function main(){
  setTimeout(()=>console.log('Hello World!'), 5000);
};
main();

What actually happens when this code is copied onto the Google Chrome browser?

1. The V8 engine will start by adding main() to the call stack.

2. Then the engine will look for the setTimeout function definition in the scope chain. (This will be available in the global scope since setTimeout is not built into the engine).

3. The V8 engine will push setTimeout() onto the call-stack, "call" the web API's setTimeout definition, which in turn will initiate whatever native implementation the browser has in place for the same, outside of Javascript's main thread.

4. At this point, as far as the engine is concerned, its job is done. Therefore, setTimeout() will be popped off the call-stack, immediately followed by main().

Okay. But that can’t be it, right? There is still a callback to be executed after a 5-second delay. Speaking about the callback, what actually happened to it?

To find these answers we need to look at the next piece of the runtime puzzle, the callback queue.

The Callback Queue

So where did that damned callback go?

Before we get to that, we need to first understand the role of the callback function.

The callback function is essentially a binding between the Javascript engine and an asynchronous web API task, usually with instructions on what to do after the asynchronous task has completed its duties. Any piece of code that runs outside of the main thread of the engine, will always require an associated callback to communicate back with the engine.

If you haven’t realized it already, this is the fundamental concept behind asynchronous Javascript.

Okay, that’s all nice and stuff, but seriously where is our callback?

Let's rewind a bit.

After the V8 engine “calls” the setTimeout web API binding, some browser-specific native code will be tasked with timing the 5-second delay. This is an asynchronous operation that happens outside of the engine's main thread, and its implementation details are not important. This same native code will also be responsible for ensuring that our "lost" callback function finds its way back home.

So how is this done?

Well, there is a special place in Javascriptland for callbacks that await execution. This is the callback queue and it is one of two crucial pieces within the Javascript runtime that makes asynchronous Javascript possible.

The native code knows of its existence and once the asynchronous task has been completed, it will push our callback function onto the callback queue.

Our callback function is almost home, but we are not quite there yet. There is still one final piece left in the Javascript runtime puzzle.

The Event Loop

What the heck is the event loop?

The event loop is the gel between the Javascript engine and the callback queue, and it does two things.

First, it will periodically check if the Javascript engine’s call-stack is empty. If it is, it will then grab a callback from the callback queue and put it onto the call stack, effectively scheduling it for execution.

It really is that simple.

Together, the callback queue and the event loop are what make your asynchronous Javascript code work. The former will queue up callback functions of asynchronous operations that are completed, while the latter will schedule them for execution on the Javascript’s main thread.

Full Circle

“…a single-threaded, non-blocking, asynchronous programming language.”

I hope this has been a fairly informative read on what actually happens under the hood of Javascript, and the above definition makes a bit more sense than what it did, a few minutes ago.

I have tried to keep things very generalized; Just to ensure that you understand that these concepts apply not just to your browser’s runtime, but to any Javascript runtime environment that you might come across.

I am also pretty sure that this will open up many new questions that you might not have had before. But that is the whole point. Questions should lead to research, which might lead to more confusion, but if you persist will eventually lead to answers. And that is how you learn.

So to summarize a whole lot of text into a few bullet points:

• The Javascript runtime is what makes your Javascript code work. It can take many different shapes and forms (browsers, node.js); But the fundamental concepts of the runtime will remain the same across all environments. The runtime environment of a browser consists of the Javascript engine, a bunch of web APIs, a callback queue, and the event loop.

• The Javascript engine converts your human-readable Javascript code into machine-readable bytecode and always executes them on a single thread. It also manages a call stack and does a bunch of other things.

• The web APIs extend the Javascript language with its own functionalities that are added to the global object of the Javascript engine. Some of these functionalities are synchronous, some can be asynchronous.

• The callback queue queues up callbacks that are awaiting to be executed by the Javascript engine. A callback is usually always associated with some asynchronous operation.

• The event loop is the gel between the Javascript engine and the callback queue. Its job is to move callbacks from the callback queue onto the engine’s call stack for execution.

• The callback queue and the event loop are at the heart of Javascript’s asynchronous (and non-blocking!) nature.

Resources

1. If you like watching rather than reading, I highly recommend checking out what the heck is the event loop anyway? by Philip Roberts. Seriously, check it out.

2. This stack overflow post discussing the link between the V8 engine and web APIs, and this one discussing the callback queue.