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CSE 154
Document Object Model (DOM)
a set of JavaScript objects that represent each element on the page
•each tag in a page corresponds to a JavaScript DOM object
•JS code can talk to these objects to examine elements' state
•e.g. see whether a box is checked
•we can change state
•e.g. insert some new text into a div
•we can change styles
•e.g. make a paragraph red
DOM element objects
• access/modify the attributes
of a DOM object
with objectName.attribute
• most DOM object attributes
have the same names as the
corresponding HTML
• img tag's src property
• a tag's href property
Accessing an element: document.getElementById
var name = document.getElementById("id"); JS
an animal
function changeImage() {
var octopusImage = document.getElementById("icon01");
octopusImage.src = "images/kitty.gif";
} JS
• document.getElementById returns the DOM object for an element with a given id
DOM object properties

See our Sales today!

var mainDiv = document.getElementById("main");
var icon = document.getElementById("icon");
var theLink = document.getElementById("saleslink"); JS
Property Description Example
tagName element's HTML tag mainDiv.tagName is "DIV"
className CSS classes of element mainDiv.className is "foo bar"
innerHTML content in element mainDiv.innerHTML is "\n

See our src URL target of an image icon.src is "images/borat.jpg"
href URL target of a link theLink.href is "sale.html"
DOM properties for form controls

var sid = document.getElementById("sid");
var frosh = document.getElementById("frosh"); JS
Property Description Example
value the text/value chosen by the user sid.value could be "1234567"
checked whether a box is checked frosh.checked is true
disabled whether a control is disabled (boolean) frosh.disabled is false
readOnly whether a text box is read-only sid.readOnly is false
More about form controls

• when talking to a text box or select, you usually want its value
• when talking to a checkbox or radio button, you probably want to know if
it's checked (true/false)
The innerHTML property

Hello HTML
function addText() {
var span = document.getElementById("output");
span.innerHTML += " bro";
} JS
• can change the text inside most elements by setting the innerHTML property
Abuse of innerHTML
// bad style!
var paragraph = document.getElementById("welcome");
paragraph.innerHTML =

text and link";
• innerHTML can inject arbitrary HTML content into the page
• however, this is prone to bugs and errors and is considered poor style
• we forbid using innerHTML to inject HTML tags; inject plain text only
• (later, we'll see a better way to inject content with HTML tags in it)
Adjusting styles with the DOM = "value"; JS

Don't forget your homework! HTML
function colorIt() {
var text = document.getElementById("fancytext"); = "#ff5500"; = "40pt";
} JS
Property Description
style lets you set any CSS style property for an element
• same properties as in CSS, but with camelCasedNames, not names-with-underscores
• examples: backgroundColor, borderLeftWidth, fontFamily
Common DOM styling errors
• many students forget to write .style when setting styles
var clickMe = document.getElementById("clickme");
clickMe.color = "red"; = "red"; JS
• style properties are capitalized likeThis, not like-this = "14pt"; = "14pt"; JS
• style properties must be set as strings, often with units at the end = 200; = "200px"; = "0.5em"; JS
• write exactly the value you would have written in the CSS, but in quotes
Unobtrusive JavaScript
• 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 no JavaScript code inside the tags
• uses the JS DOM to attach and execute all JavaScript event handlers
• 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)
// called when OK button is clicked
function okayClick() {
} JS
• 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
objectName.onevent = function; JS
var okButton = document.getElementById("ok");
okButton.onclick = okayClick; JS
• 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
When does my code run?

... HTML
var x = 3;
function f(n) { return n + 1; }
function g(n) { return n - 1; }
x = f(x); JS
• 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
• 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

var ok = document.getElementById("ok");
ok.onclick = okayClick; // error: null JS
• 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
function functionName() {
// code to initialize the page
// run this function once the page has finished loading
window.onload = functionName;
• there is a global event called window.onload event that occurs at the
moment the page body is done being loaded
• if you attach a function as a handler for window.onload, it will run at that
An unobtrusive event handler
// called when page loads; sets up event handlers
function pageLoad() {
var ok = document.getElementById("ok"); // (3)
ok.onclick = okayClick;
function okayClick() {
alert("booyah"); // (4)
window.onload = pageLoad; // (2) JS
Common unobtrusive JS errors
• event names are all lowercase, not capitalized like most variables
window.onLoad = pageLoad;
window.onload = pageLoad;
• you shouldn't write () when attaching the handler
(if you do, it calls the function immediately, rather than setting it up to be called later)
ok.onclick = okayClick();
ok.onclick = okayClick;
 our JSLint checker will catch this mistake
• related: can't directly call functions like alert; must enclose in your own function
ok.onclick = alert("booyah");
ok.onclick = okayClick;
function okayClick() { alert("booyah"); }
Anonymous functions
function(parameters) {
} JS
• 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 ok = document.getElementById("ok");
ok.onclick = okayClick;
function okayClick() {
} JS
• or the following is also legal (though harder to read and bad style):
window.onload = function() {
document.getElementById("ok").onclick = function() {
Unobtrusive styling
function okayClick() { = "red";
this.className = "highlighted";
} JS
.highlighted { color: red; } CSS
• well-written JavaScript code should contain as little CSS as possible
• use JS to set CSS classes/IDs on elements
• define the styles of those classes/IDs in your CSS file
The danger of global variables
var count = 0;
function incr(n) {
count += n;
function reset() {
count = 0;
• globals can be bad; other code and other JS files can
see and modify them
• How many global symbols are introduced by the
above code?
• 3 global symbols: count, incr, and reset
Enclosing code in a function
function everything() {
var count = 0;
function incr(n) {
count += n;
function reset() {
count = 0;
// call the function to run the code
• the above example moves all
the code into a function;
variables and functions
declared inside another
function are local to it, not
• How many global symbols are
introduced by the above
• 1 global
symbol: everything (can we
get it down to 0?)
The "module pattern"
(function() {
})(); JS
• wraps all of your file's code in an anonymous function that is declared and
immediately called
• 0 global symbols will be introduced!
• the variables and functions defined by your code cannot be messed with externally
Module pattern example
(function() {
var count = 0;
function incr(n) {
count += n;
function reset() {
count = 0;
• How many global symbols are introduced by the
above code?
• 0 global symbols
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!

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