"var" or no "var" in JavaScript's "for-in" loop?

Question

What's the correct way to write a for-in loop in JavaScript? The browser doesn't issue a complaint about either of the two approaches I show here. First, there is this approach where the iteration variable x is explicitly declared:

for (var x in set) {
    ...
}

And alternatively this approach which reads more naturally but doesn't seem correct to me:

for (x in set) {
    ...
}

Answer 1

Use var, it reduces the scope of the variable otherwise the variable looks up to the nearest closure searching for a var statement. If it cannot find a var then it is global (if you are in a strict mode, using strict, global variables throw an error). This can lead to problems like the following.

function f (){
    for (i=0; i<5; i++);
}
var i = 2;
f ();
alert (i); //i == 5. i should be 2

If you write var i in the for loop the alert shows 2.

JavaScript Scoping and Hoisting