Why the for loop counter doesn't get destroyed after exiting the loop in javascript?

Question

for(var i=0;i<5;i++){}
alert(i);

in javascript this will get us 5 other languages like C++, java, c# .... will simply give an error that the i v

Answer 1

This is because the JavaScript engine will move ("hoist") the variable decalaration to the top of the function no matter where it is declared inside the function¹. JavaScript does not have block scope.

{
//Some code
    for(var i=0;i<5;i++){}
    alert(i);
//Some code
}

Is equivalent to:

{
  var i;
 //.. some code
 for(i=0;i<5;i++){}
    alert(i);
}

_{¹ Unless it's the exception being caught with a catch clause; that variable is scoped to catch block.}

Update

For defining block scope variables ecmascript 6 specs (javascript 1.7) introduces let. Currently this will work only in latest version of FireFox browser and in consensus stage.

<script type="application/javascript;version=1.7">
     //Some code

        for (let i = 0; i < 10; i++) {

            alert(i); // 1, 2, 3, 4 ... 9
        }

        alert(i); // Here you will get an error here saying ReferenceError: i is not defined.
    }
</script>

Fiddle

Answer 2

Javascript only creates scopes for functions, with and catch blocks (with functions creating a scope for var statement), so equivalent to Java (and not working) would be:

(function(){
    for(var i=0;i<5;i++){}
})();
alert(i);

Answer 3

Variables in Javascript are subjected to var hoisting where the variable becomes declared above the block by the javascript engine.

See The Mozilla Javascript documents on var for an example of how this will work.