JavaScript Functions

JavaScript functions are used to perform operations. We can call JavaScript function many times to reuse the code.

A function is a set of statements that performs a tasks or calculates a value. Functions may also take inputs, process them and return the output.

Advantage of JavaScript function

There are mainly two advantages of JavaScript functions.

1. Code reusability: We can call a function several times so it save coding.
2. Less coding: It makes our program compact. We don’t need to write many lines of code each time to perform a common task.

JavaScript Function Syntax

The syntax of declaring function is given below.

function functionName([arg1, arg2, ...argN]){  
 //code to be executed  
}

JavaScript Functions can have 0 or more arguments.

JavaScript Function Example

Let’s see the simple example of function in JavaScript that does not has arguments.

<script>  
function msg(){  
alert("hello! this is message");  
}  
</script>  
<input type="button" onclick="msg()" value="call function"/> 

Like most programming languages, function is a piece of code that can be isolated to be called multiple times. Example of a function:

function Hello(string)
{
    return "Hello" + " " + string;
}
Hello( "Tom");  /* returns "Hello Tom" */

Function as value

Functions are values in JavaScript. A function can be assigned as a value to a variable.

    var f = function foo()
    {
        console.log("Hello");
    };
    f();  /*  returns "Hello" */

Function as argument

var f = function()
{   console.log("Hello");
};
var executor = function(fn)
{  fn();
}
executor(f);  /* returns "Hello" */
```

Function as property

var myObj = {
 "testProp" : true
};
myObj.prop1 = function() {
 console.log ("Function in object");
};
myObj.prop1();

Function arguments

var add = function()
{  var i, sum = 0;
  for (i=0; i< arguments.length; i++)
 {  sum += arguments[i];
  }
  return sum;
};

console.log(add(1,2,3,4,5));  /* 5 arguments*/
console.log(add(1,2,3,4));  /* 4 arguments*/