Bài giảng Web technologies and e-Services - Bài 3, Phần 1: JavaScript

IT4409: Web Technologies and e-Services

2020-2

JavaScript

Instructor: Dr. Thanh-Chung Dao

Slides by Dr. Binh Minh Nguyen

Department of Information Systems

School of Information and Communication Technology

Hanoi University of Science and Technology

1

Content

Client-side programming with JavaScript

§ scripts vs. programs

ØJavaScript vs. JScript vs. VBScript

Øcommon tasks for client-side scripts

§ JavaScript

Ødata types & expressions

Øcontrol statements

Øfunctions & libraries

Østrings & arrays

ØDate, document, navigator, user-defined classes

2

1

Client-Side Programming

• HTML is good for developing static pages

§ can specify text/image layout, presentation, links, …

§ Web page looks the same each time it is accessed

§ in order to develop interactive/reactive pages, must integrate programming in some form or

another

• client-side programming

§ programs are written in a separate programming (or scripting) language

e.g., JavaScript, JScript, VBScript

§ programs are embedded in the HTML of a Web page, with (HTML) tags to identify the

program component

e.g., <script type="text/javascript"> … </script>

§ the browser executes the program as it loads the page, integrating the dynamic output of

the program with the static content of HTML

§ could also allow the user (client) to input information and process it, might be used to

validate input before it’s submitted to a remote server

3

Scripts vs. Programs

• a scripting language is a simple, interpreted programming language

§ scripts are embedded as plain text, interpreted by application

§ simpler execution model: don't need compiler or development environment

§ saves bandwidth: source code is downloaded, not compiled executable

§ platform-independence: code interpreted by any script-enabled browser

§ but: slower than compiled code, not as powerful/full-featured

JavaScript: the first Web scripting language, developed by Netscape in 1995

syntactic similarities to Java/C++, but simpler, more flexible in some respects,

limited in others

(loose typing, dynamic variables, simple objects)

JScript: Microsoft version of JavaScript, introduced in 1996

same core language, but some browser-specific differences

fortunately, IE, Netscape, Firefox, etc. can (mostly) handle both

JavaScript & JScript

JavaScript 1.5 & JScript 5.0 cores both conform to ECMAScript standard

VBScript: client-side scripting version of Microsoft Visual Basic

4

2

Common Scripting Tasks

• adding dynamic features to Web pages

§ validation of form data (probably the most commonly used application)

§ image rollovers

§ time-sensitive or random page elements

§ handling cookies

• defining programs with Web interfaces

§ utilize buttons, text boxes, clickable images, prompts, etc

• limitations of client-side scripting

§ since script code is embedded in the page, it is viewable to the world

§ for security reasons, scripts are limited in what they can do

e.g., can't access the client's hard drive

§ since they are designed to run on any machine platform, scripts do not contain platform

specific commands

§ script languages are not full-featured

e.g., JavaScript objects are very crude, not good for large project development

5

JavaScript

• JavaScript code can be embedded in a Web page using <script> tags

§ the output of JavaScript code is displayed as if directly entered in HTML

<html>

document.writedisplays text in the page

<!–- CS443 js01.html 16.08.06 -->

text to be displayed can include HTML

tags

<head>

<title>JavaScript Page</title>

</head>

the tags are interpreted by the browser

when the text is displayed

<body>

// silly code to demonstrate output

document.write("<p>Hello world!</p>");

document.write(" <p>How are <br/> " +

as in C++/Java, statements end with ;

but a line break might also be interpreted as

the end of a statement (depends upon

browser)

" <i>you</i>?</p> ");

</script>

<p>Here is some static text as well.</p>

JavaScript comments similar to C++/Java

</body>

</html>

//

starts a single line comment

/*…*/enclose multi-line comments

view page

6

3

JavaScript Data Types & Variables

•

JavaScript has only three primitive data types

String : "foo" 'how do you do?'

"I said 'hi'."

""

Number: 12

3.14159

1.5E6

Boolean : true

false

*Find info on Null, Undefined

assignments are as in C++/Java

<html>

<!–- CS443 js02.html 16.08.06 -->

message = "howdy";

pi = 3.14159;

<head>

<title>Data Types and Variables</title>

</head>

variable names are sequences of letters,

digits, and underscores that start with a letter

or an underscore

<body>

var x, y;

x= 1024;

variables names are case-sensitive

y=x; x = "foobar";

document.write("<p>x = " + y + "</p>");

document.write("<p>x = " + x + "</p>");

</script>

</body>

</html>

you don't have to declare variables, will be

created the first time used, but it’s better if

you use var statements

var message, pi=3.14159;

view page

variables are loosely typed, can be assigned

different types of values (Danger!)

7

JavaScript Operators & Control Statements

standard C++/Java operators &

control statements are provided

<html>

<!–- CS443 js03.html 08.10.10 -->

<head>

in JavaScript

<title>Folding Puzzle</title>

</head>

• +, -, *, /, %, ++, --, …

• ==, !=, <, >, <=, >=

• &&, ||, !,===,!==

<body>

• if , if-else, switch

var distanceToSun = 93.3e6*5280*12;

var thickness = .002;

• while, for, do-while, …

var foldCount = 0;

while (thickness < distanceToSun) {

PUZZLE: Suppose you took a piece

of paper and folded it in half, then in

half again, and so on.

thickness *= 2;

foldCount++;

}

document.write("Number of folds = " +

foldCount);

</script>

</body>

</html>

How many folds before the thickness

of the paper reaches from the earth to

the sun?

view page

*Lots of information is available online

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4

JavaScript Math Routines

<html>

the built-in Math

object contains

functions and

constants

<!–- CS443 js04.html 08.10.10 -->

<head>

<title>Random Dice Rolls</title>

</head>

Math.sqrt

Math.pow

Math.abs

Math.max

Math.min

Math.floor

Math.ceil

Math.round

<body>

var roll1 = Math.floor(Math.random()*6) + 1;

var roll2 = Math.floor(Math.random()*6) + 1;

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

roll1 + ".gif‘ alt=‘dice showing ‘ + roll1 />");

document.write("  ");

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

Math.PI

Math.E

roll2 + ".gif‘ alt=‘dice showing ‘ + roll2 />");

</script>

</div>

Math.random

</body>

</html>

function returns a real

number in [0..1)

view page

9

Interactive Pages Using Prompt

crude user interaction can

take place using prompt

<html>

<head>

1^stargument: the prompt

message that appears in the

dialog box

<title>Interactive page</title>

</head>

<body>

var userName = prompt("What is your name?", "");

2^ndargument: a default value

that will appear in the box (in

case the user enters nothing)

var userAge = prompt("Your age?", "");

var userAge = parseFloat(userAge);

the function returns the value

entered by the user in the

dialog box (a string)

document.write("Hello " + userName + ".")

if (userAge < 18) {

document.write(" Do your parents know " +

"you are online?");

}

else {

if value is a number, must use

parseFloat (or parseInt) to

convert

document.write(" Welcome friend!");

}

</script>

</body>

</html>

forms will provide a better

interface for interaction

(later)

view page

10

5

User-Defined Functions

• function definitions are similar to C++/Java, except:

§ no return type for the function (since variables are loosely typed)

§ no variable typing for parameters (since variables are loosely typed)

§ by-value parameter passing only (parameter gets copy of argument)

function isPrime(n)

// Assumes: n > 0

// Returns: true if n is prime, else false

Can limit variable scope to the

function.

{

if (n < 2) {

return false;

}

else if (n == 2) {

return true;

if the first use of a variable is preceded

with var, then that variable is local to

the function

}

else {

for modularity, should make all

variables in a function local

for (var i = 2; i <= Math.sqrt(n); i++) {

if (n % i == 0) {

return false;

}

return true;

}

11

Function Example

<html>

<!–- CS443 js06.html 16.08.2006 -->

<head>

Function definitions

(usually) go in the

<head> section

<title>Prime Tester</title>

function isPrime(n)

// Assumes: n > 0

// Returns: true if n is prime

{

<head> section is

loaded first, so then

the function is

// CODE AS SHOWN ON PREVIOUS SLIDE

}

</script>

</head>

defined before code

in the <body> is

executed (and,

<body>

therefore, the

testNum = parseFloat(prompt("Enter a positive integer", "7"));

function can be

used later in the

body of the HTML

document)

if (isPrime(testNum)) {

document.write(testNum + " <b>is</b> a prime number.");

}

else {

document.write(testNum + " <b>is not</b> a prime number.");

}

</script>

</body>

</html>

view page

12

6

<html>

Another

<!–- CS443 js07.html 11.10.2011 -->

<head>

Example

<title> Random Dice Rolls Revisited</title>

function randomInt(low, high)

// Assumes: low <= high

// Returns: random integer in range [low..high]

{

recall the dynamic dice

page

return Math.floor(Math.random()*(high-low+1)) + low;

}

</script>

</head>

could define a function for

generating random

numbers in a range, then

use whenever needed

<body>

roll1 = randomInt(1, 6);

roll2 = randomInt(1, 6);

easier to remember,

promotes reuse

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

roll1 + ".gif'/>");

document.write("  ");

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

roll2 + ".gif'/>");

</script>

</div>

</body>

</html>

view page

13

JavaScript Libraries

• better still: if you define functions that may be useful to many pages, store in a

separate library file and load the library when needed

load a library using the SRC attribute in the SCRIPT tag (put nothing between the beginning

and ending tags)

src="random.js">

</script>

14

7

Library Example

<html>

<!–- CS443 js08.html 11.10.2011 -->

<head>

<title> Random Dice Rolls Revisited</title>

src="random.js">

</script>

</head>

<body>

roll1 = randomInt(1, 6);

roll2 = randomInt(1, 6);

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

roll1 + ".gif'/>");

document.write("  ");

document.write("<img src='http://www.csc.liv.ac.uk/"+

"~martin/teaching/CS443/Images/die" +

roll2 + ".gif'/>");

</script>

</div>

</body>

view page

</html>

15

JavaScript Objects

• an object defines a new type (formally, Abstract Data Type)

§ encapsulates data (properties) and operations on that data (methods)

• a String object encapsulates a sequence of characters, enclosed in quotes

properties include

length

: stores the number of characters in the string

methods include

charAt(index)

: returns the character stored at the given index

(as in C++/Java, indices start at 0)

: returns the part of the string between the start

(inclusive) and end (exclusive) indices

substring(start, end)

toUpperCase()

toLowerCase()

: returns copy of string with letters uppercase

: returns copy of string with letters lowercase

to create a string, assign using newor (in this case) just make a direct assignment (new is implicit)

word = new String("foo");

word = "foo";

properties/methods are called exactly as in C++/Java

word.length

word.charAt(0)

16

8

String example: Palindromes

function strip(str)

// Assumes: str is a string

// Returns: str with all but letters removed

suppose we want to

test whether a word

or phrase is a

{

var copy = "";

for (var i = 0; i < str.length; i++) {

if ((str.charAt(i) >= "A" && str.charAt(i) <= "Z") ||

palindrome

(str.charAt(i) >= "a" && str.charAt(i) <= "z")) {

copy += str.charAt(i);

noon

Radar

}

Madam, I'm Adam.

A man, a plan, a canal:

Panama!

return copy;

}

function isPalindrome(str)

// Assumes: str is a string

// Returns: true if str is a palindrome, else false

must strip non-letters out of the

word or phrase

{

str = strip(str.toUpperCase());

for(var i = 0; i < Math.floor(str.length/2); i++) {

if (str.charAt(i) != str.charAt(str.length-i-1)) {

make all chars uppercase in

order to be case-insensitive

return false;

}

finally, traverse and compare

chars from each end

return true;

}

17

<html>

<!–- CS443 js09.html 11.10.2011 -->

<head>

<title>Palindrome Checker</title>

function strip(str)

{

// CODE AS SHOWN ON PREVIOUS SLIDE

}

function isPalindrome(str)

{

// CODE AS SHOWN ON PREVIOUS SLIDE

}

</script>

</head>

<body>

text = prompt("Enter a word or phrase", "Madam, I'm Adam");

if (isPalindrome(text)) {

document.write("'" + text + "' <b>is</b> a palindrome.");

}

else {

document.write("'" + text + "' <b>is not</b> a palindrome.");

}

</script>

</body>

</html>

view page

18

9

JavaScript Arrays

• arrays store a sequence of items, accessible via an index

since JavaScript is loosely typed, elements do not have to be the same type

§ to create an array, allocate space using new (or can assign directly)

items = new Array(10);

items = new Array();

// allocates space for 10 items

// if no size given, will adjust dynamically

items = [0,0,0,0,0,0,0,0,0,0]; // can assign size & values []

§ to access an array element, use [](as in C++/Java)

for (i = 0; i < 10; i++) {

items[i] = 0;

// stores 0 at each index

}

§ the lengthproperty stores the number of items in the array

for (i = 0; i < items.length; i++) {

document.write(items[i] + "<br>");

// displays elements

}

19

Array Example

<html>

suppose we want to

simulate dice rolls and

verify even distribution

<!–- CS443 js10.html 11.10.2011 -->

<head>

<title>Dice Statistics</title>

src="http://www.csc.liv.ac.uk/~martin/teaching/CS443/JS/ran

dom.js">

</script>

</head>

<body>

keep an array of counters:

initialize each count to 0

numRolls = 60000;

each time you roll X,

increment rolls[X]

diceSides = 6;

rolls = new Array(dieSides+1);

for (i = 1; i < rolls.length; i++) {

rolls[i] = 0;

display each counter

}

for(i = 1; i <= numRolls; i++) {

rolls[randomInt(1, dieSides)]++;

}

for (i = 1; i < rolls.length; i++) {

document.write("Number of " + i + "'s = " +

rolls[i] + "<br />");

}

</script>

</body>

</html>

view page

20

10

Arrays (cont.)

• Arrays have predefined methods that allow them to be used as stacks,

queues, or other common programming data structures.

var stack = new Array();

stack.push("blue");

stack.push(12);

// stack is now the array ["blue", 12]

stack.push("green");

var item = stack.pop();

// stack = ["blue", 12, "green"]

// item is now equal to "green"

var q = [1,2,3,4,5,6,7,8,9,10];

item = q.shift(); // item is now equal to 1, remaining

// elements of q move down one position

// in the array, e.g. q[0] equals 2

q.unshift(125); // q is now the array [125,2,3,4,5,6,7,8,9,10]

q.push(244);

// q = [125,2,3,4,5,6,7,8,9,10,244]

21

Date Object

• String & Array are the most commonly used objects in JavaScript

§ other, special purpose objects also exist

• the Date object can be used to access the date and time

§ to create a Date object, use new & supply year/month/day/… as desired

today = new Date();

// sets to current date & time

newYear = new Date(2002,0,1); //sets to Jan 1, 2002 12:00AM

§ methods include:

newYear.getYear()

newYear.getMonth()

can access individual components of a date

newYear.getDay()

newYear.getHours()

newYear.getMinutes()

newYear.getSeconds()

newYear.getMilliseconds()

22

11

Date Example

<html>

<!–- CS443 js11.html 16.08.2006 -->

<head>

</head>

by default, a date will be displayed in

full, e.g.,

<body>

Time when page was loaded:

now = new Date();

Sun Feb 03 22:55:20 GMT-0600

(Central Standard Time) 2002

document.write("<p>" + now + "</p>");

time = "AM";

hours = now.getHours();

if (hours > 12) {

hours -= 12;

time = "PM"

can pull out portions of the date using

the methods and display as desired

here, determine if "AM" or "PM" and

adjust so hour between 1-12

}

else if (hours == 0) {

hours = 12;

10:55:20 PM

}

document.write("<p>" + hours + ":" +

now.getMinutes() + ":" +

now.getSeconds() + " " +

time + "</p>");

</script>

</body>

view page

</html>

23

Another Example

<html>

<!–- CS443 js12.html 12.10.2012 -->

<head>

</head>

you can add and subtract Dates:

the result is a number of

milliseconds

<body>

<p>Elapsed time in this year:

now = new Date();

here, determine the number of

seconds since New Year's day

(note: January is month 0)

newYear = new Date(2012,0,1);

secs = Math.round((now-newYear)/1000);

days = Math.floor(secs / 86400);

secs -= days*86400;

hours = Math.floor(secs / 3600);

secs -= hours*3600;

divide into number of days, hours,

minutes and seconds

minutes = Math.floor(secs / 60);

secs -= minutes*60

document.write(days + " days, " +

hours + " hours, " +

minutes + " minutes, and " +

secs + " seconds.");

</script>

</p>

</body>

view page

</html>

24

12

Document Object

Internet Explorer, Firefox, Opera, etc. allow you to access information about an

HTML document using the document object

<html>

<!–- CS443 js13.html 2.10.2012 -->

document.write(…)

<head>

method that displays text in

<title>Documentation page</title>

</head>

the page

<body>

document.URL

<tr>

property that gives the

<td><i>

location of the HTML

document

document.write(document.URL);

</script>

</i></td>

document.write(document.lastModified);

</script>

document.lastModified

property that gives the date &

time the HTML document was

last changed

</i></td>

</tr>

</table>

</body>

</html>

view page

25

User-Defined Objects

• can define new objects, but the notation can be somewhat awkward

§ simply define a function that serves as a constructor

§ specify data fields & methods using this

§ no data hiding: can't protect data or methods

// CS443

// Die class definition

////////////////////////////////////////////

Die.js

11.10.2011 //

define Diefunction (i.e.,

the object's constructor)

function Die(sides)

{

initialize data fields in the

function, preceded with

"this"

this.numSides = sides;

this.numRolls = 0;

this.roll = roll;

// define a pointer to a function

}

similarly, assign method to

separately defined function

(which uses this to access

data)

function roll()

{

this.numRolls++;

return Math.floor(Math.random()*this.numSides) + 1;

}

26

13

<html>

Object Example

<!–- CS443 js15.html 11.10.2011 -->

<head>

src="Die.js">

</script>

create a Die object using new

(similar to String and Array)

</head>

here, the argument to Die

initializes numSides for that

particular object

<body>

die6 = new Die(6);

die8 = new Die(8);

each Die object has its own

properties (numSides &

numRolls)

roll6 = -1;

roll8 = -2;

// dummy value to start loop

while (roll6 != roll8) {

roll6 = die6.roll();

roll8 = die8.roll();

Roll(), when called on a

particular Die, accesses its

numSides property and

updates its NumRolls

document.write("6-sided: " + roll6 +

"    " +

"8-sided: " + roll8 + "<br />");

}

document.write("<br />Number of rolls: " +

die6.numRolls);

</script>

</body>

</html>

view page

27

JavaScript and HTML validators

•In order to use an HTML validator, and not get error messages from the

JavaScript portions, you must “mark” the JavaScipt sections in a particular

manner. Otherwise the validator will try to interpret the script as HTML code.

•To do this, you can use a markup like the following in your inline code (this

isn’t necessary for scripts stored in external files).

// <![CDATA[

document.write(“<p>The quick brown fox jumped over the lazy dogs.</p>”);

// **more code here, etc.

// ]]>

</script>

28

14

•Since the (new) XHTML standard is written as an XML application,

validators such as the one from the W3C are actually attempting to

check an XML document for the correct structure.

•The two tags <![CDATA[ and ]]> together form an XML directive,

meaning to interpret the data between them as literal (non-parsed)

“character data”. An XML validator will effectively ignore the data

between these two tags, meaning that any symbols that would result in

an invalid document structure are ignored and do not result in an error

message from the validator.

•Because we are using these tags inside of a JavaScript block, and they

are not JavaScript commands, we precede each of them with a

(JavaScript) comment marker, hence the two forward slashes before

each tag.

29

More to learn…

• Accessing elements on the page using JavaScript functions

• JavaScript and forms

• Events, capturing user input

• The Document Object Model, and manipulating the webpage

30

15

email: chungdt@soict.hust.edu.vn

Q&A

31

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Bài giảng Web technologies and e-Services - Bài 3, Phần 1: JavaScript - Nguyễn Bình Minh