JavaScript Functions

A function is a set of statements, which perform a specific task. These functions are reusable and can be called anywhere in your program.

Functions allow a programmer to divide a big program into a number of small and manageable functions.

For example to design a program of calculator in spite of designing whole thing from starting , it is good to use small module like addition , subtraction , square root etc.

There are two type of functions in JavaScript:

Built-in Functions
User Define Functions

Built-in function are that functions which are predefined and supplied along with the compiler.

Programmer can also define functions to do a specific task relevant to their programs. Such functions are called user-defined functions. In this topic we will discuss user-defined functions.

Function Definition: Before using any function , we have to define it. We need to tell compiler about the name , return type and argument list of function.

Syntax: Following is syntax of function:

<script type = "text/javascript">
   <
      function functionname(parameter-list) {
         statements
      }
   >
</script>

Explanation:

‘function’ is a keyword, which tell the compiler that one function is defined.
‘functionname’ will be the name given by programmer.
Parameter-list is list of arguments which will pass during calling.

Example:

<script type = "text/javascript">
   <
      function first()
        {
         alert("Hello JavaScript");
        }
   >
</script

Explanation: Here we define a function named first, and it has single print statement.

Function Calling: We can call or invoke the function anywhere in program by simply to write the name of that function.

<html>
  
   <body>
         <script type = "text/javascript">
          function first1()                      //function define
          {
            document.write ("Hello javaScript");
          }
            first1();                           //function calling
         </script>
   </body>
</html>

Explanation:

Here we define a function named first1().
This function has one write statement.
We simply write the name of function and put round braces, this is the syntax to call or invoke any function.

Function Parameter: We can pass arguments into function. When we define a function we pass the list of parameters. When we call the function we pass the real values that are known as arguments. Syntax is as following:

function functionName(parameter1, parameter2, parameter3) 
{
  // code to be executed
}

Rules:

In JavaScript during function definition we need not to specify data-type parameter.
In JavaScript functions do not perform type checking on the passed arguments.
In JavaScript functions do not check the number of arguments received.

Example:

<html>
   <head> </head>
<body>   
      <script type = "text/javascript">
         function info(name, age) 
         {           
            document.write (name + " is " + age + " years old.");
                 }
     info("easecod",2);
      </script>      
   </body>
</html>

Explanation:

We define a function named info and pass two parameters(name,age).
There is single print statement.
We call info function and pass two arguments (string,integer).
name=”easecod” , age=2.

Output:

easecod is 2 years old.