13-IntroJavascript.pptxIntroduction to java script

Why use client-side
programming?
PHP already allows us to create dynamic web
pages. Why also use client-side scripting?
 client-side scripting (JavaScript) benefits:
 usability: can modify a page without having to
post back to the server (faster UI)
 efficiency: can make small, quick changes to
page without waiting for server
 event-driven: can respond to user actions like
clicks and key presses
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Why use client-side
programming?
 server-side programming (PHP) benefits:
 security: has access to server's private data;
client can't see source code
 compatibility: not subject to browser
compatibility issues
 power: can write files, open connections to
servers, connect to databases, ...
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What is Javascript?
 a lightweight programming language
("scripting language")
 used to make web pages interactive
 insert dynamic text into HTML (ex: user name)
 react to events (ex: page load user click)
 get information about a user's computer (ex:
browser type)
 perform calculations on user's computer (ex: form
validation)
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What is Javascript?
 a web standard (but not supported identically
by all browsers)
 NOT related to Java other than by name and
some syntactic similarities
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Javascript vs Java
 interpreted, not compiled
 more relaxed syntax and rules
 fewer and "looser" data types
 variables don't need to be declared
 errors often silent (few exceptions)
 key construct is the function rather than the
class
 "first-class" functions are used in many situations
 contained within a web page and integrates
with its HTML/CSS content
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Javascript vs Java
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+ =

JavaScript vs. PHP
 similarities:
 both are interpreted, not compiled
 both are relaxed about syntax, rules, and
types
 both are case-sensitive
 both have built-in regular expressions for
powerful text processing
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JavaScript vs. PHP
 differences:
 JS is more object-oriented: noun.verb(), less
procedural: verb(noun)
 JS focuses on user interfaces and interacting
with a document; PHP is geared toward HTML
output and file/form processing
 JS code runs on the client's browser; PHP
code runs on the web server
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JS <3

Linking to a JavaScript file:
script
 script tag should be placed in HTML page's
head
 script code is stored in a separate .js file
 JS code can be placed directly in the HTML
file's body or head (like CSS)
 but this is bad style (should separate content,
presentation, and behavior
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<script src="filename" type="text/javascript"></script>
HTML

Event-driven programming
12
 split breaks apart a string into an array using a
delimiter
 can also be used with regular expressions (seen
later)
 join merges an array into a single string,
placing a delimiter between them
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A JavaScript statement: alert
 a JS command that pops up a dialog box with
a message
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alert("IE6 detected. Suck-mode enabled.");
JS

Event-driven programming
14
 you are used to programs start with a main
method (or implicit main like in PHP)
 JavaScript programs instead wait for user
actions called events and respond to them
 event-driven programming: writing programs
driven by user events
 Let's write a page with a clickable button that
pops up a "Hello, World" window...
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Buttons
 button's text appears inside tag; can also
contain images
 To make a responsive button or other UI
control:
1. choose the control (e.g. button) and event (e.g.
mouse 1. click) of interest
2. write a JavaScript function to run when the
event occurs
3. attach the function to the event on the control
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<button>Click me!</button> HTML

JavaScript functions
16
function name() {
statement ;
statement ;
...
statement ;
} JS
 the above could be the contents of example.js
linked to our HTML page
 statements placed into functions can be
evaluated in response to user events
function myFunction() {
alert("Hello!");
alert("How are you?");
} JS
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Event handlers
 JavaScript functions can be set as event
handlers
 when you interact with the element, the function will
execute
 onclick is just one of many event HTML
attributes we'll use
 but popping up an alert window is disruptive and
annoying
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<element attributes onclick="function();">...
HTML
<button onclick="myFunction();">Click me!</button>
HTML

Document Object Model (DOM)
 most JS code
manipulates elements on
an HTML page
 we can 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
18

Accessing elements:
document.getElementById
20
var name = document.getElementById("id");
JS
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<button onclick="changeText();">Click me!</button>
<span id="output">replace me</span>
<input id="textbox" type="text" /> HTML
function changeText() {
var span = document.getElementById("output");
var textBox = document.getElementById("textbox");
textbox.style.color = "red";
} JS

Accessing elements:
document.getElementById
21
 document.getElementById returns the DOM
object for an element with a given id
 can change the text inside most elements by
setting the innerHTML property
 can change the text in form controls by setting
the value property
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Changing element style:
element.style
22
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Attribute Property or style object
color color
padding padding
background-color backgroundColor
border-top-width borderTopWidth
Font size fontSize
Font famiy fontFamily

Preetify
23
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function changeText() {
//grab or initialize text here
// font styles added by JS:
text.style.fontSize = "13pt";
text.style.fontFamily = "Comic Sans MS";
text.style.color = "red"; // or pink?
} JS

More Javascript Syntax
24
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Variables
 variables are declared with the var keyword
(case sensitive)
 types are not specified, but JS does have
types ("loosely typed")
 Number, Boolean, String, Array, Object,
Function, Null, Undefined
 can find out a variable's type by calling typeof
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var name = expression; JS
var clientName = "Connie Client";
var age = 32;
var weight = 127.4; JS

Number type
 integers and real numbers are the same type
(no int vs. double)
 same operators: + - * / % ++ -- = += -= *= /=
%=
 similar precedence to Java
 many operators auto-convert types: "2" * 3 is 6
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var enrollment = 99;
var medianGrade = 2.8;
var credits = 5 + 4 + (2 * 3);
JS

Comments (same as Java)
 identical to Java's comment syntax
 recall: 4 comment syntaxes
 HTML: 
 CSS/JS/PHP: /* comment */
 Java/JS/PHP: // comment
 PHP: # comment
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// single-line comment
/* multi-line comment */
JS

Math object
28
var rand1to10 = Math.floor(Math.random() * 10 + 1);
var three = Math.floor(Math.PI);
JS
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 methods: abs, ceil, cos, floor, log,
max, min, pow, random, round, sin,
sqrt, tan
 properties: E, PI

Special values: null and
undefined
29
var ned = null;
var benson = 9;
// at this point in the code,
// ned is null
// benson's 9
// caroline is undefined
JS
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 undefined : has not been declared, does not
exist
 null : exists, but was specifically assigned an
empty or null value
 Why does JavaScript have both of these?

Logical operators
30
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 > < >= <= && || ! == != === !==
 most logical operators automatically convert
types:
 5 < "7" is true
 42 == 42.0 is true
 "5.0" == 5 is true
 === and !== are strict equality tests; checks
both type and value
 "5.0" === 5 is false

if/else statement (same as
Java)
31
if (condition) {
statements;
} else if (condition) {
statements;
} else {
statements;
}
JS
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 identical structure to Java's if/else statement
 JavaScript allows almost anything as a
condition

Boolean type
32
var iLike190M = true;
var ieIsGood = "IE6" > 0; // false
if ("web devevelopment is great") { /* true */ }
if (0) { /* false */ }
JS
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 any value can be used as a Boolean
 "falsey" values: 0, 0.0, NaN, "", null, and
undefined
 "truthy" values: anything else
 converting a value into a Boolean explicitly:
 var boolValue = Boolean(otherValue);
 var boolValue = !!(otherValue);

for loop (same as Java)
33
var sum = 0;
for (var i = 0; i < 100; i++) {
sum = sum + i;
} JS
var s1 = "hello";
var s2 = "";
for (var i = 0; i < s.length; i++) {
s2 += s1.charAt(i) + s1.charAt(i);
}
// s2 stores "hheelllloo" JS
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while loops (same as Java)
34
while (condition) {
statements;
} JS
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 break and continue keywords also behave as
in Java
do {
statements;
} while (condition);
JS

Popup boxes
35
alert("message"); // message
confirm("message"); // returns true or false
prompt("message"); // returns user input string
JS
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Arrays
36
var name = []; // empty array
var name = [value, value, ..., value]; // pre-filled
name[index] = value; // store element
JS
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var ducks = ["Huey", "Dewey", "Louie"];
var stooges = []; // stooges.length is 0
stooges[0] = "Larry"; // stooges.length is 1
stooges[1] = "Moe"; // stooges.length is 2
stooges[4] = "Curly"; // stooges.length is 5
stooges[4] = "Shemp"; // stooges.length is 5
JS

Array methods
37
var a = ["Stef", "Jason"]; // Stef, Jason
a.push("Brian"); // Stef, Jason, Brian
a.unshift("Kelly"); // Kelly, Stef, Jason, Brian
a.pop(); // Kelly, Stef, Jason
a.shift(); // Stef, Jason
a.sort(); // Jason, Stef
JS
 array serves as many data structures: list,
queue, stack, ...
 methods: concat, join, pop, push, reverse,
shift, slice, sort, splice, toString, unshift
 push and pop add / remove from back
 unshift and shift add / remove from front
 shift and pop return the element that is removed

String type
 methods: charAt, charCodeAt, fromCharCode,
indexOf, lastIndexOf, replace, split,
substring, toLowerCase, toUpperCase
 charAt returns a one-letter String (there is no char
type)
 length property (not a method as in Java)
 Strings can be specified with "" or ''
 concatenation with + :
38
var s = "Connie Client";
var fName = s.substring(0, s.indexOf(" ")); // "Connie"
var len = s.length; // 13
var s2 = 'Melvin Merchant';
JS

More about String
 accessing the letters of a String:
39
var count = 10;
var s1 = "" + count; // "10"
var s2 = count + " bananas, ah ah ah!"; // "10 bananas, ah
ah ah!"
var n1 = parseInt("42 is the answer"); // 42
var n2 = parseFloat("booyah"); // NaN JS
 escape sequences behave as in Java: ' " &
n t
 converting between numbers and Strings:
var firstLetter = s[0]; // fails in IE
var firstLetter = s.charAt(0); // does work in IE
var lastLetter = s.charAt(s.length - 1); JS
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Splitting strings: split and join
40
var s = "the quick brown fox";
var a = s.split(" "); // ["the", "quick", "brown", "fox"]
a.reverse(); // ["fox", "brown", "quick", "the"]
s = a.join("!"); // "fox!brown!quick!the"
JS
 split breaks apart a string into an array using a
delimiter
 can also be used with regular expressions (seen
later)
 join merges an array into a single string,
placing a delimiter between them

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