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JavaScript - Global Variables

JavaScript Global Variables

The global variables in JavaScript are the variables that are defined outside of the function or any particular block. They are accessible from anywhere in JavaScript code. All scripts and functions can access the global variables.

You can define the global variables using the var, let, or const keyword. The variables defined without using any of the var, let or const keywords automatically becomes global variables.

JavaScript Global Scope

The global variables have global scope. So a variable declared outside of a function or block has global scope. The global scope is visible or accessible in all other scope. In client side JavaScript the global scope is the web page in which all the code is being executed. A global variable declared with var keyword belongs to window object.

var x = 10; // Global Scope
let y = 20; // Global Scope	
const z = 30; // Global Scope

Here the variables, x, y and z are declared outside of any function and block, so they have global scope and are called global variable.

Global Variable Examples

Let's learn more about global variables using the example below.

Example

We have defined the x, y, and z global variables in the code below. You can observe that variables can be accessed anywhere inside the code.

<html>
<head>
   <title> JavaScript - Global variables </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      var x = 10;
      let y = 20;
      const z = 30;
      document.getElementById("output").innerHTML = 
	  "x = " + x + "<br>" +
	  "y = " + y + "<br>" +
      "z = " + z;
   </script>
</body>
</html>

Output

x = 10
y = 20
z = 30

Example

In the example below, we have defined the variables a and b using the var and let keywords. You can see that a and b variables can be accessible inside the function or outside the function as they are global variables.

<html>
<head>
   <title> JavaScript - Global variables </title>
</head>
<body>
   <p id = "demo"> </p>
   <script>
      const output = document.getElementById("demo");
      var a = 10;
      let b = 20;
      function test() {
         output.innerHTML += "a -> Inside the function = " + a + "<br>";
         output.innerHTML += "b -> Inside the function = " + b + "<br>";
      }
      test();
      output.innerHTML += "a -> Outside the function = " + a + "<br>";
      output.innerHTML += "b -> Outside the function = " + b + "<br>";
   </script>
</body>
</html>

Output

a -> Inside the function = 10
b -> Inside the function = 20
a -> Outside the function = 10
b -> Outside the function = 20

Automatic Global Variables

When you define the variables anywhere inside the code without using the var, let, or const keywords, the variable automatically becomes the global variable and can be accessible anywhere inside the code.

Example

In the below code, we have defined the variable 'a' without using any keyword inside the function. Even if we have defined the variable in the function, it becomes global as we haven't used any keyword to define the function.

The output shows that variable 'a' can be accessible inside or outside the function.

<html>
<head>
   <title> JavaScript - Global variables </title>
</head>
<body>
   <p id = "demo"> </p>
   <script>
      const output = document.getElementById("demo");
      function test() {
      a = 90;
            output.innerHTML += "a -> Inside the function = " + a + "<br>";
        }
        test();
        output.innerHTML += "a -> Outside the function = " + a + "<br>";
    </script>
</body>
</html>

Output

a -> Inside the function = 90
a -> Outside the function = 90

Defining the global variables inside the function or particular block is not a good practice, as naming conflict can occur in the code.

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