The following are pre-defined functions in JavaScript.
1. isNaN ( )
2. toUpperCase( )
3. toLowerCase( )
4. length( )
5. alertt( )
6. promptt ( )
7. write( )

Pre-defined functions:
Pre-defined functions are already defined in the JavaScript library which are also called Library functions.

(i) The is NaN ( ) function is used to check whether the given value or variable is valid number.
(ii) This function returns true if the given value is not a number. For example isNaN("12"), is NaN ("A").

The function-name is any valid identifier.
(i) For Example: sum
(ii) The parameter list contains one or more valid variable name.
(iii) Parameter list contains more than one variable then comma must be there between the variable.
(iv) The function body must be enclosed by braces.

JavaScript supports two types functions. They are
(i) Pre-defined or Library Functions.
(ii) User-defined Functions.

The format of a functions definition is
Function function-name(parametets list)

User-defined functions allow the programmer to modularize a program. Most computer programs that solve real-world problems are much large, occupy more space in the computer memory and takes more time to execute. Hence such large programs are divided into small programs called modules.

Functions are used to encapsulate code that performs. a specific task. Sometimes functions are defined for commonly required tasks to avoid the repetition entailed in typing the same statements over and over. More generally, they are used to keep the code that performs a particular job in one place in order to enhance reusability and program clarity.

< html >
< body >
< script language = "JavaScript" type "text/ JavaScript" >
var N;
var N = prompt("Enter the N terms", "O");
var x = parse Int(N);
var s = sum(x);
document.write in("The sum of 10 numbers = "+8);
function sum (x)
{
var s = 0;
var s = (x*(x + 1))/2;
return s ;
}
return
< /script >
< /body >
< /html >

Program to find the cube of a number:
< html >
< body >
< script language = "JavaScript" type = "text/ JavaScript" >
var N = window.prompt("Enter the number to find cube" ,"O")',
var x = parse Int(N);
vary s = cube (x);
document, written ("The cube of a number" + x + "is" +s); function cube(m)
{
var s = m*m*m;
returns s ;
}
< /script >
< /body >
< /html >

HTML CODE
< html >
< head >
< title >On-line Qulze-c/title >
< script type="text/JavaScript":;'\
function checkAnswer ( )
{
var myQuiz = document.getElementByld
("myQuiz'') ;
if . ( document.getElementById("myQuiz'').
elements[O] .checked)
alert("Congratulations, Your Answer is correct'');
else
alert("Your Answer is incorrect, Pleasetry Again'');
}
< /script >
< /head >
< body >
< form id="myQuiz" action="JavaScript:check
Answert ( )" >
< p > Which is not a Programming
Language:. < br >
< input type="radio" name="radiobutton"
value=''Word'' / >
< label > MS-Word< /Iabel >
< input type="radio" name="radiobutton"
value="Cobol" / >
< label > COBOL< /Iabel >
< input type="radio" name="radiobutton"
value="CPP" / >
C++< /Iabel >
< input type="radio" name="radiobutton"
value=''VB''  / >
< label >Visual BASIC < /Iabel >< br >< br >
< input type="submit" name="submit"
value="Submit" / >
< input type="reset"
value="Reset" / >
< /p >
< /form >
< /body >
< /htm l>

HTMLCODE
< html>
< title > Example Program to test isNan() Function < title >
< head>
< body.>
< h4 >< u >Example Program to test isNan() Function< /u >< /h4 >
< script language="JavaScript" >
function checknum()
{
var n=documentform1.text1.value;
if(isNaN(n)==true)
{
documentform1.text2.value="Not a
Number: "+n;
}
else
{
documentJorm1.text2.value='1t is
Number: "+n;
}
}
< /scrip t>
< form name="form1" >
Enter a Number 1:
< input type="text" name="text1" size=3 >
< br >< br >
< input type="button" value="Click to Check" onClick= "checknum()" >
< input type="text" name="text2" size=30 >
< br >
< /form >
< /body >
< /html >

< html >
< title >Example Program to test isNan( ) Function< /title >
< head > < /head >
< body >
< h4 >< u >Example Program to test isNan() Function
< /u >< /h4 >
< script language=" JavaScript" >
function checknum( )
{
var n=document.form 1.text1.value;
if(isNaN(n)=true)
{
document.form1.text2.value="Not a Number:"+n;
}
else
{
document.forml.text2.value="It is Number: "+n;
}
}
< /script >
< form name="forml" >
Enter a Number1:
< input type="text" name=rtext1" size=3 >
< br >< br >
< input type=t'button" value="Click to Check" onClick="checknum( )" >
< input type="text" name="text2" size=30 >
< br >
< /form >
< /body >
< /html >

< html >
< head >
< title > Function Example < /title >
< script type = "text/JavaScript'' >
var input 1 = window.prompt("Enter Valuel :", "0");
var input2=Window.prompuEnter Value2:","0");
var v1=parselnt(input1);
var v2=parselnt(input2);
var s=sum(v1,v2);
document. writeln(" < br > < h4 >< u > Example for
Function< /u >< /h4 >");
document. writeln("First No: "+v 1+< br > Second
No:"+ v2+"< br >The Sum = " + s);
function sum(x, y)
{
var s=x+y;
return s;
}
< /script >
< /head >
< body >
< /body >
< /html >