Programming for beginners: global variable

In case of web pages, window is the global object. If you assign a value to any undeclared variable, then it becomes the property of the global object window.

For example,

Hello.html

<!DOCTYPE html>

<html>
 <head>
  <title>Variables</title>
 </head>
 
 <body>
  <script type="text/javascript">
 message = "Hello World";
 
 document.write("message : " + window.message);
  </script>
 </body>
</html>

As you see, the variable ‘message’ is undeclared (not declared with var, let, const keywords), but assigned with value "Hello World". In this case, it becomes the property of global object ‘window’.

What is the advantage of Global variables that are attached to global object?

You can access the global variable that is declared in one frame (or) window from another frame (or) window.

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Saturday, 8 December 2018

How to declare a global variable in a document?

When you declare any variable outside of a function, then it is global to the current document, and all the code in the document can access this global variable.

HelloWorld.js

var x = 10;

function increment_x(){
  x++;
}

function multiply_x_by_10(){
  x *= 10;
}

function print_x(){
  console.log("Value of x is : " + x);
}

increment_x();
print_x();

multiply_x_by_10();
print_x();

Output

Value of x is : 11

Value of x is : 110

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