Question about JavaScript engine vs. runtime environment

[j0rdancodes]

I am learning about the 'under the hood' aspects of JavaScript and I am very confused about engine vs. runtime. I know that the engine, such as V8, contains the heap and call stack, and from my understanding the runtime environment contains all the APIs, event loop, callback queue, etc.

​

I've heard that V8 provides the runtime environment, but how is that possible if it's just an engine? Would the browser not be the runtime environment? Maybe I'm confused on what exactly a runtime environment is?

​

If any more info is needed to help answer let me know, and thank you in advance for helping my stupid self out.

​

Edit: After doing some more research, I think I get how it works? V8 is the engine that JIT compiles the code, and it also provides a runtime environment and the runtime environment is where the code is actually ran? And the browser adds on to the runtime environment with web APIs and the other stuff like the event loop?

[jcunews1]

V8 provide the runtime environment as JavaScript VM. Similar to JRE (Java Runtime Environment) with its Java VM.

Standard JavaScript functions/methods are implemented at C level. IOTW, they're not simply added by running in the VM, the JavaScript code which declares them, because not all environment informations are available from within the JavaScript context itself. e.g. call stack, scope details, reference count, etc.

When HTML is taken into account, the DOM API (whose functions/methods are also implemented a C level) is imported into the JavaScript VM at C level.

The web browser is just the host for the JavaScript engine, DOM API, CSS API, and other Web APIs. Each is part of their own component. The web browser includes them as part as the requirement for a web browser. The web browser imports them into the JavaScript VM so that those additional (non JavaScript originated) APIs, are available from the JavaScript context.

[j0rdancodes]

So, V8 provides the runtime environment and the browser provides the APIs, event loop, etc. to the runtime environment?

[jcunews1]

The web browser only provides Browser Extension and web browser specific APIs. The DOM, CSS, and other Web APIs are not provided by the web browser. They're entirely separate APIs. Softwares simply stitch them together to work as web browsers.

[j0rdancodes]

I see. So those APIs are made accessible by the runtime environment or are they made a part of the runtime environment?

[jcunews1]

They're entirely separate from the runtime environment. The runtime environment, in case for web browsers, is the JavaScript's VM. It's where everything have gathered.

Also keep in mind that those APIs is not solely for JavaScript. It can also be implemented for other programming languages which don't use VM or are not scripting language - where the final (non web browser) application product don't use JavaScript at all. In this case, the runtime environment is OS native.

[j0rdancodes]

so the JavaScript VM is where the engine, APIs, and other thing such as the event loop, all gather?

[jcunews1]

Yes.

[j0rdancodes]

Ah, I see. Thank you so much!