How to define global variable in a JavaScript function?

In JavaScript, you can define global variables either at global scope or from within functions using the window object. Here are the main approaches.

Method 1: Declaring at Global Scope

The most straightforward way is to declare variables outside any function at global scope:

<script>
   var myGlobalVariable = "I'm global!";
   
   function display() {
      console.log(myGlobalVariable); // Accessible here
   }
   
   display();
   console.log(myGlobalVariable); // Also accessible here
</script>

I'm global!
I'm global!

Method 2: Using window Object Inside Functions

You can create global variables from within functions by assigning to the window object:

<script>
   function createGlobal() {
      window.myGlobalVar = "Created inside function";
   }
   
   createGlobal();
   console.log(myGlobalVar); // Accessible globally
   console.log(window.myGlobalVar); // Same variable
</script>

Created inside function
Created inside function

Method 3: Omitting var/let/const (Not Recommended)

Variables declared without var, let, or const become global, but this is not recommended:

<script>
   function createImplicitGlobal() {
      implicitGlobal = "Accidentally global"; // No var/let/const
   }
   
   createImplicitGlobal();
   console.log(implicitGlobal); // Works but not recommended
</script>

Accidentally global

Comparison

Key Points

Global variables are accessible from anywhere in your code but can lead to naming conflicts. Use them sparingly and prefer module patterns or proper scoping when possible.

Conclusion

Declare global variables at global scope for clarity, or use window object inside functions when needed. Avoid implicit globals to prevent bugs and maintain clean code.