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.

Pre-defined functions are already defined in the JavaScript library which are also called Library functions. For example is NaN(), to UpperCase(), to LowerCase(), length(), alert(), prompt(), write() etc., are pre-defined functions.

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.

A function is a block ofJavaScript code that is defined once but may be executed or invoked any number of times.

< 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 >