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 variable isn't defined in the context.

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

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