Non-JavaScript

So far, the only things we’ve covered are in the JS language itself. The reality is that most JS is written to run in and interact with environments like browsers. A good chunk of the stuff that you write in your code is, strictly speaking, not directly controlled by JavaScript. That probably sounds a little strange.

The most common non-JavaScript JavaScript you’ll encounter is the DOM API. For example:

var el = document.getElementById( "foo" );

The document variable exists as a global variable when your code is running in a browser. It’s not provided by the JS engine, nor is it particularly controlled by the JavaScript specification. It takes the form of something that looks an awful lot like a normal JS object, but it’s not really exactly that. It’s a special object, often called a “host object.”

Moreover, the getElementById(..) method on document looks like a normal JS function, but it’s just a thinly exposed interface to a built-in method provided by the DOM from your browser. In some (newer-generation) browsers, this layer may also be in JS, but traditionally the DOM and its behavior is implemented in something more like C/C++.

Another example is with input/output (I/O).

Everyone’s favorite alert(..) pops up a message box in the user’s browser window. alert(..) is provided to your JS program by the browser, not by the JS engine itself. The call you make sends the message to the browser internals and it handles drawing and displaying the message box.

The same goes with console.log(..); your browser provides such mechanisms and hooks them up to the developer tools.

This book, and this whole series, focuses on JavaScript the language. That’s why you don’t see any substantial coverage of these non-JavaScript JavaScript mechanisms. Nevertheless, you need to be aware of them, as they’ll be in every JS program you write!