Browser environment, specs

The JavaScript language was initially created for web browsers. Since then, it evolved and became a language with many uses and platforms.

A platform may be either a browser or a web-server or a washing machine or another host. Each of them provides platform-specific functionality. The JavaScript specification calls that a host environment.

A host environment provides platform-specific objects and functions additionally to the language core. Web browsers give means to control web pages. Node.JS provides server-side features, and so on.

Here’s a bird-eye view of what we have when JavaScript runs in a web-browser:

There’s a “root” object called window. It has two roles:

  1. First, it is a global object for JavaScript code, as described in the chapter Global object.
  2. Second, it represents the “browser window” and provides methods to control it.

For instance, here we use it as a global object:

function sayHi() {

// global functions are accessible as properties of window

And here we use it as a browser window, to see the window height:

alert(window.innerHeight); // inner window height

There are more window-specific methods and properties, we’ll cover them later.

Document Object Model (DOM)

The document object gives access to the page content. We can change or create anything on the page using it.

For instance:

// change the background color to red = 'red';

// change it back after 1 second
setTimeout(() => = '', 1000);

Here we used, but there’s much, much more. Properties and methods are described in the specification. By chance, there are two working groups who develop it:

  1. W3C – the documentation is at
  2. WhatWG, publishing at

As it happens, the two groups don’t always agree, so we have like 2 sets of standards. But they are in the tight contact and eventually things merge. So the documentation that you can find on the given resources is very similar, there’s like 99% match. There are very minor differences, you probably won’t notice them.

Personally, I find more pleasant to use.

In the ancient past, there was no standard at all – each browser implemented whatever it wanted. So different browsers had different sets methods and properties for the same thing, and developers had to write different code for each of them. Dark, messy times.

Even now we can sometimes meet old code that uses browser-specific properties and works around incompatibilities. But in this tutorial we’ll use modern stuff: there’s no need to learn old things until you really need those (chances are high you won’t).

Then the DOM standard appeared, in an attempt to bring everyone to an agreement. The first version was “DOM Level 1”, then it was extended by DOM Level 2, then DOM Level 3, and now it’s DOM Level 4. People from WhatWG group got tired of version and are calling that just “DOM”, without a number. So will do we.

DOM is not only for browsers

The DOM specification explains the structure of a document and provides objects to manipulate it. There are non-browser instruments that use it too.

For instance, server-side tools that download HTML pages and process them. They may support only a part of the DOM specification though.

CSSOM for styling

CSS rules and stylesheets are structured not like HTML. So there’s a separate specification CSSOM that explains how they are represented as objects, how to read and write them.

CSSOM is used together with DOM when we modify style rules for the document. In practice though, CSSOM is rarely required, because usually CSS rules are static. We rarely need to add/remove CSS rules from JavaScript, so we won’t cover it right now.

BOM (part of HTML spec)

Browser Object Model (BOM) are additional objects provided by the browser (host environment) to work with everything except the document.

For instance:

  • navigator object provides background information about the browser and the operation system. There are many properties, but two most widely known are: navigator.userAgent – about the current browser, and navigator.platform – about the platform (can help to differ between Windows/Linux/Mac etc).
  • location object allows to read the current URL and redirect the browser to a new one.

Here’s how we can use the location object:

alert(location.href); // shows current URL
if (confirm("Go to wikipedia?")) {
  location.href = ''; // redirect the browser to another URL

Functions alert/confirm/prompt are also a part of BOM: they are directly not related to the document, but represent pure browser methods of communicating with the user.

HTML specification

BOM is the part of the general HTML specification.

Yes, you heard that right. The HTML spec at is not only about the “HTML language” (tags, attributes), but also covers a bunch of objects, methods and browser-specific DOM extensions. That’s “HTML in broad terms”.


Talking about standards, we have:

DOM specification
Describes the document structure, manipulations and events, see
CSSOM specification
Describes stylesheets and style rules, manipulations with them and their binding to documents, see
HTML specification
Describes HTML language (tags etc) and also BOM (browser object model) – various browser functions: setTimeout, alert, location and so on, see It takes DOM specification and extends it with many additional properties and methods.

Now we’ll get down to learning DOM, because the document plays the central role in the UI, and working with it is the most complex part.

Please note the links above, because there’s so many stuff to learn, it’s impossible to cover and remember everything.

When you’d like to read about a property or a method – the Mozilla manual at is a nice one, but reading the corresponding spec may be better: more complex and longer to read, but will make your fundamental knowledge sound and complete.

Tutorial map


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