Collision of Global and Local Variables

JavaScript Tutorials - Herong's Tutorial Examples

∟Collision of Global and Local Variables - Examples

This section provides a tutorial example on how JavaScript handles local variables collide with global variables.

To see JavaScript handles global and local variables collide with identical names inside and outside functions, I wrote the following tutorial example:

<html>
<!-- Variable_Collision.html
   Copyright (c) 2008 HerongYang.com. All Rights Reserved.
-->
<head><title>Variable Collision</title></head>
<body>
<pre>
<script type="text/javascript">
   var tempVar;
   tempVar = "Bird";
   tempNoVar = "Fish";
  
   scopeCheck();

   document.write("\n\nAfter function call:");
   document.write("\n   tempVar = " + tempVar);
   document.write("\n   tempNoVar = " + tempNoVar);

function scopeCheck() {
   var tempVar;
   tempVar = "Bird";
   tempNoVar = "Fish";

   tempVar = tempVar + " - Updated";
   tempNoVar = tempNoVar + " - Updated";

   document.write("\n\nUpdated value in function:");
   document.write("\n   tempVar = " + tempVar);
   document.write("\n   tempNoVar = " + tempNoVar);
}
</script>
</pre>
</body>
</html>

This tutorial example tests two sets variables. One set introduced outside the function, and one set inside the function. Names of variables in one set collide with the other set. Two "tempVar" declared with "var" statements stay as two variables. But two "tempNoVar" auto-declared without "var" statements become one variable. See the output:

Updated value in function:
   tempVar = Bird - Updated
   tempNoVar = Fish - Updated

After function call:
   tempVar = Bird
   tempNoVar = Fish - Updated