JavaScript
Content
* Introduction to Javascript
* Where to write script block
* Javascript Statements
* Javascript Comments
* Variables
* Data Types
* Arrays
* Alert box
* Confirm box
* Prompt box
Introduction to Javascript
* JavaScript is used in web pages to add functionality, validate forms, communicating with server and read write html elements.
* JavaScript is a client side scripting language.
* JavaScript is an interpreted language, rather than compiled language.
* As the browser goes through the JavaScript, it passes it to a special program called interpreter, which converts the JavaScript to the machine code your computer understands
* Browsers come equipped with JavaScript interpreters.
* Initially it was known as LiveScript
* It was first available with Netscape Navigator 2.
* Netscape decided to change its name to JavaScript as Java was the hot technology of the time.
* Following is the script tag for javascript
<script type="text/javascript">
...
...
</script>
Where to write Script Block
* Either between <head> and </head> or between <body> and </body>
* If we want to run the <script> before page loads then it is kept in the <head> section.
* If we want to run the <script> after the page loads so that it can interact with HTML elements then it is kept in the <body> section.
* JavaScript can also be placed in external files.
* External script cannot contain the <script></script> tags
* <script type="text/javascript" src="myscript.js"> </script>
JS Statements
* JavaScript is a sequence of statements to be executed by the browser.
* JavaScript is case sensitive language.
* Example
<script type="text/javascript">
document.write("JavaScript Example");
</script>
* document is an object
* write is a function or method used to print message on web page
* It is normal to add a semicolon at the end of each executable statement
* However, semicolon is optional according to the JavaScript standards
JS Comments
* Comments can be added to explain the JavaScript, or to make the code more readable.
* Single line comments start with //.
* Multi line comments start with /* and end with */.
Variables
* JavaScript variables are used to hold values or expressions.
* Variable names are case sensitive.
* Variable names must begin with a letter or the underscore character
* A variables value can change during the execution of a script
* Declaring JavaScript variable with var keyword
* var x;
The above declaration creates a variable name x and contains nothing.
However we can initialize variable during declaration.
* var x=2;
If you redeclare a JavaScript variable, it will not lose its value.
* When you assign a text value to a variable, use quotes around the value.
* var y="saurabh";
If you assign values to variables that have not yet been declared, the variables will automatically be declared as global variables
* N="Saurabh";
Important note
* Every non zero value is true, 0 is false
* You can compare two strings unlike C language
Example
<script type="text/javascript">
var s1="amit";
var s2="amar";
if(s1==s2)
document.write("Strings are same");
else
document.write("Strings are not same");
</script>
Data Type
* Javascript has dynamic types, this means that the same variable can be used as different types
* You can assign boolean values
* var x=true;
* var x=false;
Displaying Variable value
To display a variables value we just have to pass it’s name to the document.write( ) method.
var a=10;
document.write(a);
To print a variable’s value with some text we use operator " + "
var a=10;
document.write(‘Value of a is ‘+a);
Arrays
* var students= new Array();
students[0]="Amit";
students[1]="Mita";
students[2]="bunty";
* var students= new Array("Amit","Mita","bunty");
* var students= ["Amit","Mita","bunty"];
* You can always use length property of array
* For example in the above case students.length is 3.
Alert Box
* An alert box is often used if you want to make sure information comes through the user
* When an alert box pops up, the user will have to click "OK" to proceed.
* alert("some text");
Example
<script type="text/javascript">
alert("Welcome to home page");
</script>
Confirm Box
* A confirm box is often used if you want the user to verify or accept something.
* When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed
* If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false
* variable=confirm("some text");
* Variable would get either true or false as a value.
Example
<script type="text/javascript">
var x = confirm("Are you interested in learning?");
if(x)
document.write("Good, you are a geek");
else
document.write("Ok, clap for others!!!");
</script>
Prompt Box
* A prompt box is often used if you want the user to input a value before entering a page.
* When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value
* If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null
* var x=prompt("some text", "default value");
Example
<html>
<head>
<title> JAVASCRIPT </title>
<script type="text/javascript" >
var x=parseInt(prompt("Enter a number","0"));
var i=1;
while(i<=10)
{
a=i*x;
document.write(x+" x "+i+" = "+a+"<br/>");
++i;
}
</script>
</head>
<body>
</body>
</html>
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Saturday, July 1, 2017
Java Script
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