It is said, javascript clears a variable from memory after its being referenced last.

just for the sake of this question i created a JS file with only one variable;

//file start 

//variable defined
var a=["Hello"]

//refenence to that variable
alert(a[0]);

//
//file end

no further reference to that variable, so i expect javascript to clear varaible 'a'

Now i just ran this page and then opened firebug and ran this code

alert(a[0]);

Now this alerts the value of variable, If the statement "Javascript clears a variable after there is no further reference it" is true how come alert() shows its value.

Is it because all variable defined in global context become properties of window object, and since even after the execution file window objects exist so does it properties.