Web Programming - Introduction to Java Script - Lecture Slides, Slides of Javascript programming

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Web Programming

client-side programming with JavaScript

 scripts vs. programs

 JavaScript vs. JScript vs. VBScript

 common tasks for client-side scripts

 JavaScript

 data types & expressions

 control statements

 functions & libraries

 strings & arrays

 Date, document, navigator, user-defined classes

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.,

 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

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

docsity.com

• adding dynamic features to Web pagesCommon Scripting Tasks
  • 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

JavaScript

• JavaScript code can be embedded in a Web page using

  Here is some static text as well.

  document.write displays text in the page

  text to be displayed can include HTML

  tags

  the tags are interpreted by the browser

  when the text is displayed

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

  but a line break might also be interpreted as

  the end of a statement (depends upon

  browser)

  JavaScript comments similar to C++/Java

  // starts a single line comment

  /…/ enclose multi-line comments view page

  JavaScript Data Types & Variables

  • JavaScript has only three primitive data types

  String : "foo" 'how do you do?' "I said 'hi'." ""

  Number : 12 3.14159 1.5E

  Boolean : true false *Find info on Null, Undefined

  assignments are as in C++/Java

  message = "howdy"; pi = 3.14159;

  variable names are sequences of letters,

  digits, and underscores that start with a letter

  or an underscore

  variables names are case-sensitive

  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;

  variables are loosely typed, can be assigned

  different types of values (Danger!)

  view page

  JavaScript Operators & Control Statements

  standard C++/Java operators &

  control statements are provided

  in JavaScript

  • +, - , *, /, %, ++, --, …
  • ==, !=, <, >, <=, >=
  • &&, ||, !,===,!==
  • if , if-else, switch
  • while, for, do-while, …

  PUZZLE: Suppose you took a piece

  of paper and folded it in half, then in

  half again, and so on.

  How many folds before the thickness

  of the paper reaches from the earth to

  the sun?

  *Lots of information is available online

  view page

  JavaScript Math Routines

  the built-in Math

  object contains

  functions and

  constants

  Math.sqrt Math.pow Math.abs Math.max

  Math.min Math.floor Math.ceil Math.round

  Math.PI Math.E

  Math.random

  function returns a real

  number in [0..1)

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  Interactive Pages Using Prompt

  The rest of the page...

  crude user interaction can

  take place using prompt

  1 st^ argument: the prompt

  message that appears in the

  dialog box

  2 nd^ argument: a default value

  that will appear in the box (in

  case the user enters nothing)

  the function returns the value

  entered by the user in the

  dialog box (a string)

  if value is a number, must use

  parseFloat (or parseInt) to

  convert

  forms will provide a better

  interface for interaction

  (later)

  view page

  • function definitions are similar to C++/Java, except:User-Defined Functions
    • 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 { if (n < 2) { return false; } else if (n == 2) { return true; } else { for (var i = 2; i <= Math.sqrt(n); i++) { if (n % i == 0) { return false; } } return true; } }

  Can limit variable scope to the

  function.

  if the first use of a variable is preceded

  with var, then that variable is local to

  the function

  for modularity, should make all

  variables in a function local

  Function Example

  Function definitions

  (usually) go in the

  section

  section is

  loaded first, so then

  the function is

  defined before code

  in the is

  executed (and,

  therefore, the

  function can be

  used later in the

  body of the HTML

  document)

  view page

  Another

  Example

  recall the dynamic dice

  page

  could define a function for

  generating random

  numbers in a range, then

  use whenever needed

  easier to remember,

  promotes reuse

  view page

  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

  • the file at http://www.csc.liv.ac.uk/~martin/teaching/comp519/JS/random.js

  contains definitions of the following functions:

  randomNum(low, high) returns random real in range [low..high) randomInt(low, high) returns random integer in range [low..high) randomChar(string) returns random character from the string randomOneOf([item1,…,itemN]) returns random item from list/array

  Note: as with external style sheets, do not put

  Library Example

  view page

  • an object defines a new type (formally,JavaScript Objects 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)
  • substring(start, end) : returns the part of the string between the start
  • (inclusive) and end (exclusive) indices
  • toUpperCase() : returns copy of string with letters uppercase
  • toLowerCase() : returns copy of string with letters lowercase

  to create a string , assign using new or (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)

  String example: Palindromes function strip(str)

  // Assumes: str is a string

  // Returns: str with all but letters removed

  {

  var copy = ""; for (var i = 0; i < str.length; i++) { if ((str.charAt(i) >= "A" && str.charAt(i) <= "Z") || (str.charAt(i) >= "a" && str.charAt(i) <= "z")) { copy += str.charAt(i); } } return copy;

  }

  function isPalindrome(str)

  // Assumes: str is a string

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

  {

  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)) { return false; } } return true;

  }

  suppose we want to

  test whether a word

  or phrase is a

  palindrome

  noon Radar

  Madam, I'm Adam.

  A man, a plan, a canal:

  Panama!

  must strip non-letters out of the word or phrase

  make all chars uppercase in

  order to be case-insensitive

  finally, traverse and compare chars from each end

  view page

  • arrays store a sequence of items, accessible via an indexJavaScript Arrays

  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); // allocates space for 10 items
    • items = new Array(); // 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 length property stores the number of items in the array
    • for (i = 0; i < items.length; i++) {
    • document.write(items[i] + "
      "); // displays elements
    • }

  Array Example

  suppose we want to

  simulate die rolls and

  verify even distribution

  keep an array of counters:

  initialize each count to 0

  each time you roll X,

  increment rolls[X]

  display each counter

  view page

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

  var item = stack.pop(); // 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]

  • String & Array are the most commonly used objects in JavaScriptDate Object
    • 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() can access individual components of a date
      • newYear.getMonth()
      • newYear.getDay()
      • newYear.getHours()
      • newYear.getMinutes()
      • newYear.getSeconds()
      • newYear.getMilliseconds()

  Date Example

  Time when page was loaded:

  by default, a date will be displayed in

  full, e.g.,

  Sun Feb 03 22:55:20 GMT- 0600 (Central Standard Time) 2002

  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

  10:55:20 PM

  view page

  Another Example

  Elapsed time in this year:

  you can add and subtract Dates:

  the result is a number of

  milliseconds

  here, determine the number of

  seconds since New Year's day

  (note: January is month 0)

  divide into number of days, hours,

  minutes and seconds

  view page

  document Object

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

  HTML document using the document object

  document.write(…)

  method that displays text in

  the page

  document.URL

  property that gives the

  location of the HTML

  document

  document.lastModified

  property that gives the date &

  time the HTML document was

  last changed

  view page

  navigator Object (^)

  Here is some text with a link.

  a {text-decoration:none; font-size:larger; color:red; font-family:Arial} a:hover {color:blue}

  a {font-family:Arial; color:white; background-color:red}

  navigator.appName

  property that gives the browser

  name

  navigator.appVersion

  property that gives the browser

  version

  view page