An introduction to javascript

INTRODUCTION TO WEB
PROGRAMMING
Lecture 16: JavaScript

Copyright Notice
 The materials (used by permission) in this
presentation are from Web Programming Step by
Step written by Marty Stepp, Jessica Miller, and
Victoria Kirst.
 http://www.webstepbook.com/supplements-2ed.shtml
 Document's original contents are © Copyright 2012
Marty Stepp, Jessica Miller, and Victoria; revised by
Anthony Hetrick. 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.

JavaScript
1. Purposes of JavaScript
2. Basic language structure and syntax
3. Event-Driven Programming with JavaScript

Client-side scripting
 client-side script: code runs in
browser after page is sent back from server
 often this code manipulates the page or responds to
user actions

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

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, ...

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)
 a web standard (but not supported identically by all
browsers)
 NOT related to Java other than by name and some

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

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

JavaScript vs. PHP
 differences:
 JS is more object-oriented: noun.verb(), less
procedural: verb(noun)
 JS focuses on UIs and interacting with a
document; PHP on HTML output and files/forms
 JS code runs on the client's browser; PHP code
runs on the web server

JavaScript
1. Purposes of JavaScript
2. Basic language structure and syntax
3. Event-Driven Programming with
JavaScript

A JavaScript statement: alert
alert("message"); JS

alert("Hello, I'm an alert message!"); JS

 a JS command that pops up a dialog box with
a message

Variables and types
var name = expression; JS

var name = expression;
var age = 32;
var weight = 127.4;
var clientName = "Connie Client"; JS

 variables are declared with the var keyword (case
sensitive)
 types are not specified, but JS does have types ("loosely
typed")

Number type
var enrollment = 99;
var medianGrade = 2.8;
var credits = 5 + 4 + (2 * 3); JS

 integers and real numbers are the same type
(no int vs. double)
 same operators: + - * / % ++ -- = += -= *= /= %=
 many operators auto-convert types: "2" *
3 is 6

String type
var s = "Connie Client";
var fName = s.substring(0, s.indexOf(" ")); // "Connie"
var len = s.length; // 13
var s2 = 'Melvin Merchant'; // can use "" or ' ' JS

 methods: charAt, charCodeAt, fromCharCode, indexO
f, lastIndexOf, replace, split, substring,toLowerCase, t
oUpperCase
 charAt returns a one-letter String (there is no char type)
 length property (not a method as in Java)
 concatenation with + : 1 + 1 is 2, but "1" + 1 is "11"

More about String
 escape sequences behave as in Java:
' " & n t
 to convert between numbers and Strings:
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

More about String
 to access characters of a String, use
[index] or charAt:

var firstLetter = s[0];
var firstLetter = s.charAt(0);
var lastLetter = s.charAt(s.length - 1); JS

Comments
// single-line comment

/* multi-line comment */ JS

 identical to Java's comment syntax
 recall: 4 comment syntaxes
 HTML: 
 CSS/JS/PHP: /* comment */
 Java/JS/PHP: // comment
 PHP: # comment

for loop
for (initialization; condition; update) {
statements;
} JS

var s1 = "hello";
var s2 = "";
for (var i = 0; i < s.length; i++) {
s2 += s1[i] + s1[i];
}
// s2 stores "hheelllloo" JS

Math object
var rand1to10 = Math.floor(Math.random() * 10 + 1);
var three = Math.floor(Math.PI); JS

 methods: abs, ceil, cos, floor, log, max, min, po
w, random, round, sin, sqrt, tan
 properties: E, PI

Logical operators
 > < >= <= && || ! == != === !==
 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
if (condition) {
statements;
} else if (condition) {
statements;
} else {
statements;
} JS

 JavaScript allows almost anything as
a condition

Boolean type
var iLike190M = true;
var ieIsGood = "IE6" > 0; // false
if ("web dev is great") { /* true */ } JS
if (0) { /* false */ }

 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);

while loops
while (condition) {
statements;
} JS

do {
statements;
} while (condition) JS

 JavaScript also has keywords
break and continue

Arrays
var name = []; // empty array
var name = [value, value, ..., value]; // pre-filled
name[index] = value; // store elementJS

 two ways to initialize an array
 length property (grows as needed when
elements are added)

Arrays
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

 two ways to initialize an array
 length property (grows as needed when
elements are added)

Array methods
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, ...

Array methods
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

 methods: concat, join, pop, push, reverse, shift, slice, sort, spl
ice,
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

Splitting strings: split and join
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 surrounded
by /:
var a = s.split(/[ t]+/);
 join merges an array into a single string, placing
a delimiter between them

Defining functions
function name() {
statement ;
statement ;
...
statement ;
} JS

function myFunction() {
alert("Hello!");
alert("How are you?");
} JS

Special values: null and undefined
var ned = null; // ned is null
var benson = 9; // benson's 9
var caroline; // caroline is undefined JS

 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?

Linking to a JavaScript file: script
<script src="filename.js" type="text/javascript"></script>
HTML

 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)

Event-driven programming
 JS programs have no main; they respond to user
actions called events
 event-driven programming: writing programs
driven by user events

Buttons: <button>
<button>Click me!</button>
HTML

 button's text appears inside tag; can also contain
images
 To make a responsive button or other UI control:
 choose the control (e.g. button) and event (e.g.
mouse click) of interest
 write a JavaScript function to run when the event
occurs
 attach the function to the event on the control

Event handlers
<element attributes onclick="function();">...
HTML

<button onclick="myFunction();">Click me!</button>
HTML

 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

An introduction to javascript

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