What happens when I try to loop on an int like this:
var x = 0;
for (var k in x) {
// x[k]
}
Should I look for unexpected behavior or does it just not enter the loop? (so far my experiments seem to show that it does not enter the loop... but it could do something I don't see)
There are no methods defined in the vanilla JavaScript Number object prototype. Your code enters the loop (all objects in JS can be iterated) but there is nothing to iterate over in this case.
If you would do:
Number.prototype.foo = "bar";
Your loop body would run once for the foo property.
console.dir(0) says "no properties").console.dir(new Number(0)) works though (__proto__ shows contains these methods).x isn't a collection. What is there for it to iterate through?
Only objects can be iterated with the for in loop.
So, say you have a variable a that holds a reference to something. If typeof a !== 'object' then you cannot iterate over your a var.
Note that you can iterate over arrays, as arrays are objects too (alert(typeof [])).
As for your example, x is not an object, it is a number (typeof x == 'number'), so the for in will not work; but if you declare your number as a Number object :
var x = new Number(0);
you can loop over its methods since now typeof x == 'object'
Number object in this case.