Web Programming Step by Step

Lecture 15
Unobtrusive JavaScript

Reading: 8.1 - 8.3

8.1: Global DOM Objects

8.1: Global DOM Objects
8.2: DOM Element Objects
8.3: The DOM Tree

The six global DOM objects

Every Javascript program can refer to the following global objects:

name	description
`document`	current HTML page and its content
`history`	list of pages the user has visited
`location`	URL of the current HTML page
`navigator`	info about the web browser you are using
`screen`	info about the screen area occupied by the browser
`window`	the browser window

The `window` object

the entire browser window; the top-level object in DOM hierarchy

technically, all global code and variables become part of the window object
properties:
- document, history, location, name
methods:
- alert, confirm, prompt (popup boxes)
- setInterval, setTimeout clearInterval, clearTimeout (timers)
- open, close (popping up new browser windows)
- blur, focus, moveBy, moveTo, print, resizeBy, resizeTo, scrollBy, scrollTo

The `document` object

the current web page and the elements inside it

properties:
- anchors, body, cookie, domain, forms, images, links, referrer, title, URL
methods:
complete list

The `location` object

the URL of the current web page

properties:
- host, hostname, href, pathname, port, protocol, search
methods:
- assign, reload, replace
complete list

The `navigator` object

information about the web browser application

properties:
- appName, appVersion, browserLanguage, cookieEnabled, platform, userAgent
- complete list
Some web programmers examine the navigator object to see what browser is being used, and write browser-specific scripts and hacks:
```
if (navigator.appName === "Microsoft Internet Explorer") { ...
```
- (this is poor style; you should not need to do this)

The `screen` object

information about the client's display screen

properties:
- availHeight, availWidth, colorDepth, height, pixelDepth, width
- complete list

The `history` object

the list of sites the browser has visited in this window

properties:
- length
methods:
- back, forward, go
complete list
sometimes the browser won't let scripts view history properties, for security

Unobtrusive JavaScript (8.1.1)

JavaScript event code seen previously was obtrusive, in the HTML; this is bad style
now we'll see how to write unobtrusive JavaScript code
- HTML with minimal JavaScript inside
- uses the DOM to attach and execute all JavaScript functions
allows separation of web site into 3 major categories:
- content (HTML) - what is it?
- presentation (CSS) - how does it look?
- behavior (JavaScript) - how does it respond to user interaction?

Obtrusive event handlers (bad)

<button id="ok" onclick="okayClick();">OK</button>

// called when OK button is clicked
function okayClick() {
	alert("booyah");
}

this is bad style (HTML is cluttered with JS code)
goal: remove all JavaScript code from the HTML body

Attaching an event handler in JavaScript code

// where element is a DOM element object
element.event = function;

$("ok").onclick = okayClick;

it is legal to attach event handlers to elements' DOM objects in your JavaScript code
- notice that you do not put parentheses after the function's name
this is better style than attaching them in the HTML
Where should we put the above code?

When does my code run?

	<head>
		<script src="myfile.js" type="text/javascript"></script>
	</head>

	<body> ... </body>

// global code
var x = 3;
function f(n) { return n + 1; }
function g(n) { return n - 1; }
x = f(x);

your file's JS code runs the moment the browser loads the script tag
- any variables are declared immediately
- any functions are declared but not called, unless your global code explicitly calls them
at this point in time, the browser has not yet read your page's body
- none of the DOM objects for tags on the page have been created yet

A failed attempt at being unobtrusive

	<head>
		<script src="myfile.js" type="text/javascript"></script>
	</head>

	<body>
		<div><button id="ok">OK</button></div>

// global code
$("ok").onclick = okayClick;   // error: $("ok") is null

problem: global JS code runs the moment the script is loaded
script in head is processed before page's body has loaded
- no elements are available yet or can be accessed yet via the DOM
we need a way to attach the handler after the page has loaded...

The `window.onload` event (8.1.1)

// this will run once the page has finished loading
function functionName() {
	element.event = functionName;
	element.event = functionName;
	...
}

window.onload = functionName;   // global code

we want to attach our event handlers right after the page is done loading

there is a global event called window.onload event that occurs at that moment

in window.onload handler we attach all the other handlers to run when events occur

An unobtrusive event handler

<!-- look Ma, no JavaScript! -->
<button id="ok">OK</button>

// called when page loads; sets up event handlers
function pageLoad() {
	$("ok").onclick = okayClick;
}

function okayClick() {
	alert("booyah");
}

window.onload = pageLoad;  // global code

Common unobtrusive JS errors

many students mistakenly write () when attaching the handler

window.onload = pageLoad();
window.onload = pageLoad;

okButton.onclick = okayClick();
okButton.onclick = okayClick;

our JSLint checker will catch this mistake

event names are all lowercase, not capitalized like most variables
```
window.onLoad = pageLoad;
window.onload = pageLoad;
```

Anonymous functions (8.1.2)

function(parameters) {
	statements;
}

JavaScript allows you to declare anonymous functions
quickly creates a function without giving it a name
can be stored as a variable, attached as an event handler, etc.

Anonymous function example

window.onload = function() {
	var okButton = document.getElementById("ok");
	okButton.onclick = okayClick;
};

function okayClick() {
	alert("booyah");
}

or the following is also legal (though harder to read and bad style):

window.onload = function() {
	var okButton = document.getElementById("ok");
	okButton.onclick = function() {
		alert("booyah");
	};
};

The keyword `this` (8.1.3)

this.fieldName                  // access field
this.fieldName = value;          // modify field

this.methodName(parameters);    // call method

all JavaScript code actually runs inside of an object
by default, code runs inside the global window object
- all global variables and functions you declare become part of window
the this keyword refers to the current object

Event handler binding

function pageLoad() {
	$("ok").onclick = okayClick;   // bound to okButton here
}

function okayClick() {           // okayClick knows what DOM object
	this.innerHTML = "booyah";     // it was called on
}

window.onload = pageLoad;

event handlers attached unobtrusively are bound to the element
inside the handler, that element becomes this (rather than the window)

Fixing redundant code with `this`

<fieldset>
	<label><input type="radio" name="ducks" value="Huey"  /> Huey</label>
	<label><input type="radio" name="ducks" value="Dewey" /> Dewey</label>
	<label><input type="radio" name="ducks" value="Louie" /> Louie</label>
</fieldset>

function processDucks() {
	if ($("huey").checked) {
		alert("Huey is checked!");
	} else if ($("dewey").checked) {
		alert("Dewey is checked!");
	} else {
		alert("Louie is checked!");
	}
	alert(this.value + " is checked!");
}

if the same function is assigned to multiple elements, each gets its own bound copy

Web Programming Step by Step, Lecture 15

Unobtrusive JavaScript