Global in JavaScript

Global scope is a JavaScript function Scope.Scope determines the accessibility (visibility) of variables.

Variables defined inside a function are not accessible (visible) from outside the function.

A global variable is declared outside the function or declared with an object. It can be accessed from any function.

Declaring global variable within function

To declare global variables within a function, we need to use window object. For example:

window.value=90;

1	window.value=90;

Now this value can declared with in any function and can be access and used from any function.

For example:

function m(){  
	window.value=100;//declaring global variable by window object  
}  
function n(){  
	alert(window.value);//accessing global variable from other function  
}

function m(){

window.value=100;//declaring global variable by window object

}

function n(){

alert(window.value);//accessing global variable from other function

}

EXAMPLE

<html>

<body>
    <script>
        function m() {
            window.value = 100; //declaring global variable by window object  
        }

        function n() {
            alert(window.value); //accessing global variable from other function  
        }
        m();
        n();
    </script>
</body>

</html>

<html>

<body>

function m() {

window.value = 100; //declaring global variable by window object

}

function n() {

alert(window.value); //accessing global variable from other function

}

m();

n();

</script>

</body>

</html>

OUTPUT

100

100

Internals of global variable

On outside a function,when a variable declare, that will add in window object. So it can be access through window object.

For example:

<html>

<body>
    <script>
        var value = 50;

        function a() {
            alert(window.value); //accessing global variable   
        }
    </script>
    <input type="button" onclick="a()" value="get global"/>
</body>

</html>

<html>

<body>

var value = 50;

function a() {

alert(window.value); //accessing global variable

}

</script>

</body>

</html>

Global JavaScript Variables

If any variable declared outside a function, so that will becomes GLOBAL.

A global variable has global scope: All scripts and functions on a web page can access it.

<!DOCTYPE html>
<html>

<body>

    <p id="jsdemo"></p>

    <script>
        var myName = "Ayan";
        showName();

        function showName() {
            document.getElementById("jsdemo").innerHTML = "My name is " + myName;
        }
    </script>
</body>

</html>

<!DOCTYPE html>

<html>

<body>

var myName = "Ayan";

showName();

function showName() {

document.getElementById("jsdemo").innerHTML = "My name is " + myName;

}

</script>

</body>

</html>

OUTPUT

My name is Ayan

1	My name is Ayan

Automatically Global

If a value assign to a variable that has not been declared, it will automatically turn on GLOBAL variable.

This example will declare a global variable myName, the value is assigned inside a function.

<!DOCTYPE html>
<html>
<body>

<p id="jsdemo"></p>

<script>
myFunction();
document.getElementById("jsdemo").innerHTML = "My name is " + myName;
function myFunction() {
myName = "Ayan";
}
</script>

</body>
</html>

<!DOCTYPE html>

<html>

<body>

myFunction();

document.getElementById("jsdemo").innerHTML = "My name is " + myName;

function myFunction() {

myName = "Ayan";

}

</script>

</body>

</html>

OUTPUT

My name is Ayan

1	My name is Ayan

Global Variables in HTML

With JavaScript, the global scope is the complete
JavaScript environment.

In HTML, the global scope is the window object. All global variables belong to the window object.

<!DOCTYPE html>
<html>

<body>

    <p id="jsdemo"></p>

    <script>
        var myName = "Ayan";

        document.getElementById("jsdemo").innerHTML = "My name is " + window.myName;
    </script>

</body>

</html>

<!DOCTYPE html>

<html>

<body>

var myName = "Ayan";

document.getElementById("jsdemo").innerHTML = "My name is " + window.myName;

</script>

</body>

</html>

OUTPUT

My name is Ayan

1	My name is Ayan

Global Properties

Property	Description
Infinity	A numeric value that represents positive/negative infinity
NaN	“Not-a-Number” value, It means there is nothing like value.
undefined	This property indicates that a variable will not assigned a value.

Global Functions

Function	Description
decodeURI()	It will decodes a URI
decodeURIComponent()	It will decodes a URI component
encodeURI()	It will encode a URI
encodeURIComponent()	It will encode a URI component
escape()	It will use encodeURI() or encodeURIComponent() instead
eval()	It willevaluates a string and executes it as if it was script code.
isFinite()	It will determine whether a value is a finite, normal number
Number()	It will convert an object’s value to a number
parseFloat()	It will parse a string and returns a floating point number
parseInt()	It will parse a string and returns an integer
String()	It will convert an object’s value to a string
unescape()	It will use decodeURI() or decodeURIComponent() instead