Valid, just doesn't do what you think.

function foo() { var x = 1; if (true) { var x = 2; var y = 3; } console.log(x); console.log(y); }

foo(); // 2, 3 (vs 1, undefined if there was block scope)

Thanks for the explanation!

JS performs "variable hoisting", so your example is interpreted as:

    function(){ var i; … for(i=0; …

It has surprising side effects:

    alert(i);
    var i;

works and outputs "undefined", which is value of the variable i that is defined.

    alert(k);
    var i;

This is an error, and will fail to execute because of undefined variable.