JavaScript Documentation

Browser environment, specs

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

A platform may be a browser, or a web-server or another host, or even a “smart” coffee machine if it can run JavaScript. Each of these provides platform-specific functionality. The JavaScript specification calls that a host environment.

A host environment provides its own objects and functions in addition to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on.

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

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

First, it is a global object for JavaScript code

Second, it represents the “browser window” and provides methods to control it.

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

function sayHi() {
    alert("Hello");
}
                  
// global functions are methods of the global object:
window.sayHi();

And we can use it as a browser window, to show the window height:

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

DOM (Document Object Model)

The Document Object Model, or DOM for short, represents all page content as objects that can be modified.

The document object is the main “entry point” to the page. We can change or create anything on the page using it.

For instance

// change the background color to red
document.body.style.background = "red";

// change it back after 1 second
setTimeout(() => document.body.style.background = "", 1000);

The DOM specification explains the structure of a document and provides objects to manipulate it. There are non-browser instruments that use DOM too. For instance, server-side scripts that download HTML pages and process them can also use the DOM. They may support only a part of the specification though.

BOM (Browser Object Model)

The Browser Object Model (BOM) represents additional objects provided by the browser (host environment) for working with everything except the document.

For instance:

The navigator object provides background information about the browser and the operating system. There are many properties, but the two most widely known are: navigator.userAgent – about the current browser, and navigator.platform – about the platform (can help to differentiate between Windows/Linux/Mac etc).

The location object allows us to read the current URL and can 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 = "https://wikipedia.org"; // redirect the browser to another URL
}

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