Web Programming Step by Step, 2nd Edition

Lecture 16: The DOM Tree

Reading: 8.2.6, 9.3–9.4

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

9.3: The DOM Tree

9.1: Global DOM Objects
9.2: DOM Element Objects
9.3: The DOM Tree

Complex DOM manipulation problems

How would we do each of the following in JavaScript code? Each involves modifying each one of a group of elements ...

When the Go button is clicked, reposition all the divs of class puzzle to random x/y locations.
When the user hovers over the maze boundary, turn all maze walls red.
Change every other item in the ul list with id of TAs to have a gray background.

The DOM tree

The elements of a page are nested into a tree-like structure of objects
- the DOM has properties and methods for traversing this tree

Selecting groups of DOM objects

methods in document and other DOM objects:

name	description
`getElementsByTagName`	returns array of descendents with the given tag, such as `"div"`
`getElementsByName`	returns array of descendents with the given `name` attribute (mostly useful for accessing form controls)
`querySelector`	returns the first element that would be matched by the given CSS selector string
`querySelectorAll`	returns an array of all elements that would be matched by the given CSS selector string

Getting all elements of a certain type

highlight all paragraphs in the document:

var allParas = document.querySelectorAll("p");
for (var i = 0; i < allParas.length; i++) {
	allParas[i].style.backgroundColor = "yellow";
}

<body>
	<p>This is the first paragraph</p>
	<p>This is the second paragraph</p>
	<p>You get the idea...</p>
</body>

Complex selectors

highlight all paragraphs inside of the section with ID "address":

// document.getElementById("address").getElementsByTagName("p")
var addrParas = document.querySelectorAll("#address p");
for (var i = 0; i < addrParas.length; i++) {
	addrParas[i].style.backgroundColor = "yellow";
}

<p>This won't be returned!</p>
<div id="address">
	<p>1234 Street</p>
	<p>Atlanta, GA</p>
</div>

Common `querySelectorAll` issues

many students forget to write . or # in front of a class or id

// get all buttons with a class of "control"
var gameButtons = document.querySelectorAll("control");
var gameButtons = document.querySelectorAll(".control");

querySelectorAll returns an array, not a single element; must loop over the results (document.querySelector returns just the first element that matches, if that's what you want)

// set all buttons with a class of "control" to have red text
document.querySelectorAll(".gamebutton").style.color = "red";
var gameButtons = document.querySelector(".gamebutton");
for (var i = 0; i < gameButtons.length; i++) {
	gameButtons[i].style.color = "red";
}

Q: Can I still select a group of elements using querySelectorAll even if my CSS file doesn't have any style rule for that same group? (A: Yes!)

Creating new nodes

name	description
`document.createElement("tag")`	creates and returns a new empty DOM node representing an element of that type
`document.createTextNode("text")`	creates and returns a text node containing given text

// create a new <h2> node
var newHeading = document.createElement("h2");
newHeading.innerHTML = "This is a heading";
newHeading.style.color = "green";

merely creating a element does not add it to the page
you must add the new element as a child of an existing element on the page...

Modifying the DOM tree

Every DOM element object has these methods:

name	description
`appendChild(node)`	places given node at end of this node's child list
`insertBefore(new, old)`	places the given new node in this node's child list just before `old` child
`removeChild(node)`	removes given node from this node's child list
`replaceChild(new, old)`	replaces given child with new node

var p = document.createElement("p");
p.innerHTML = "A paragraph!";
document.getElementById("main").appendChild(p);

DOM versus `innerHTML` hacking

Why not just code the previous example this way?

function slideClick() {
	document.getElementById("main").innerHTML += "<p>A paragraph!</p>";
}

Imagine that the new node is more complex:
- ugly: bad style on many levels (e.g. JS code embedded within HTML)
- error-prone: must carefully distinguish " and '
- can only add at beginning or end, not in middle of child list

function slideClick() {
	document.getElementById("main").innerHTML += "<p style='color: red; " +
			"margin-left: 50px;' " +
			"onclick='myOnClick();'>" +
			"A paragraph!</p>";
}

Removing a node from the page

function slideClick() {
	var bullet = document.getElementById("removeme");
	bullet.parentNode.removeChild(bullet);
}

each DOM object has a removeChild method to remove its children from the page
odd idiom: obj.parentNode.remove(obj);

Problems with reading/changing styles

<button id="clickme">Click Me</button>

window.onload = function() {
	document.getElementById("clickme").onclick = biggerFont;
};
function biggerFont() {
	var button = document.getElementById("clickme");
	var size = parseInt(button.style.fontSize);
	button.style.fontSize = (size + 4) + "pt";
}

style property lets you set any CSS style for an element
problem: you cannot read existing styles with it
(you can read ones you set using the DOM .style, but not ones that are set in the CSS file)

Accessing elements' existing styles

window.getComputedStyle(element).propertyName

function biggerFont() {
	// turn text yellow and make it bigger
	var clickMe = document.getElementById("clickme");
	var size = parseInt(window.getComputedStyle(clickMe).fontSize);
	clickMe.style.fontSize = (size + 4) + "pt";
}

getComputedStyle method of global window object accesses existing styles

Common bug: incorrect usage of existing styles

the following example computes e.g. "200px" + 100 + "px" ,
which would evaluate to "200px100px"

var main = document.getElementById("main");
main.style.top = window.getComputedStyle(main).top + 100 + "px";            // bad!

a corrected version:

main.style.top = parseInt(window.getComputedStyle(main).top) + 100 + "px";  // correct

Getting/setting CSS classes

function highlightField() {
	// turn text yellow and make it bigger
	var text = document.getElementById("text");
	if (!text.className) {
		text.className = "highlight";
	} else if (text.className.indexOf("invalid") < 0) {
		text.className += " highlight";   // awkward
	}
}

JS DOM's className property corresponds to HTML class attribute
somewhat clunky when dealing with multiple space-separated classes as one big string

Getting/setting CSS classes with `classList`

function highlightField() {
	// turn text yellow and make it bigger
	var text = document.getElementById("text");
	if (!text.classList.contains("invalid")) {
		text.classList.add("highlight");
	}
}

classList collection has methods add, remove, contains, toggle to manipulate CSS classes
similar to existing className DOM property, but don't have to manually split by spaces

Types of DOM nodes

<p>
	This is a paragraph of text with a 
	<a href="/path/page.html">link in it</a>.
</p>

element nodes (HTML tag)
- can have children and/or attributes
text nodes (text in a block element)
attribute nodes (attribute/value pair)
- text/attributes are children in an element node
- cannot have children or attributes
- not usually shown when drawing the DOM tree

Traversing the DOM tree manually

every node's DOM object has the following properties:

name(s)	description
`firstChild`, `lastChild`	start/end of this node's list of children
`childNodes`	array of all this node's children
`nextSibling`, `previousSibling`	neighboring nodes with the same parent
`parentNode`	the element that contains this node

complete list of DOM node properties
browser incompatiblity information (IE6 sucks)

DOM tree traversal example

<p id="foo">This is a paragraph of text with a 
	<a href="/path/to/another/page.html">link</a>.</p>

Element vs. text nodes

<div>
	<p>
		This is a paragraph of text with a 
		<a href="page.html">link</a>.
	</p>
</div>

Q: How many children does the div above have?
A: 3
- an element node representing the <p>
- two text nodes representing "\n\t" (before/after the paragraph)
Q: How many children does the paragraph have? The a tag?

8.2.6: Debugging Common Errors

8.1: Key JavaScript Concepts
8.2: JavaScript Syntax
8.3: Program Logic
8.4: Advanced JavaScript Syntax

Debugging JS code in Firebug

Firebug/Chrome JS debugger can set breakpoints, step through code, examine values (Script tab)
interactive console for typing in arbitrary JS expressions (Console tab)

JSLint

JSLint: an analyzer that checks your JS code, much like a compiler, and points out common errors
- Web Step By Step version (recommended)
- original version, by Douglas Crockford of Yahoo!
when your JS code doesn't work, paste it into JSLint first to find many common problems

JavaScript "strict" mode

"use strict";

your code...

writing "use strict"; at the very top of your JS file turns on strict syntax checking:
- shows an error if you try to assign to an undeclared variable
- stops you from overwriting key JS system libraries
- forbids some unsafe or error-prone language features
You should always turn on strict mode for your code in this class!

Web Programming Step by Step, 2nd Edition

Lecture 16: The DOM Tree

Web Programming Step by Step, 2nd Edition

Lecture 16: The DOM Tree

Reading: 8.2.6, 9.3&ndash;9.4

9.3: The DOM Tree

Complex DOM manipulation problems

The DOM tree

Selecting groups of DOM objects

Getting all elements of a certain type

Complex selectors

Common querySelectorAll issues

Creating new nodes

Modifying the DOM tree

DOM versus innerHTML hacking

Removing a node from the page

Problems with reading/changing styles

Accessing elements' existing styles

Common bug: incorrect usage of existing styles

Getting/setting CSS classes

Getting/setting CSS classes with classList

Types of DOM nodes

Traversing the DOM tree manually

DOM tree traversal example

Element vs. text nodes

8.2.6: Debugging Common Errors

Debugging JS code in Firebug

JSLint

JavaScript "strict" mode

Reading: 8.2.6, 9.3–9.4

Common `querySelectorAll` issues

DOM versus `innerHTML` hacking

Getting/setting CSS classes with `classList`