“in” can also be used in an expression, i.e.

if (‘index’ in { index: 0 }) // do stuff
else // do other stuff

@Aaron : Thanks for the correction. I was too lazy to write code without an additional “,” at the end.

Thanks also to be such a great commenter. Even if your comments are most of the time caught by Akismet as spam

var properties = [];
for(var name in obj) {
properties.push(name);
}
alert(properties.join(“, “);

Will probably get you what you want. If you have Firebug, you could also use console.log to get a message in the log, instead of a popup dialog. (I really can’t emphasize enough the amazingness of _not_ having to alert() your log messages)

Of course, you could also combine the two for-loops into:

var properties = [];
var functions = [];

for(var name in obj) {
if(obj[name] instanceof Function)
functions.push(name);
else
properties.push(name);
}

You also probably’d want to join() the stuff in alertClass, otherwise you’ll end up with “The class The full name of the class51″. (By alert([this.name, this.fullName, this.age].join(” “)); )

One important thing that I mentioned in another post is this very common bug with for(var i in obj).

Let’s say you want to sum some numbers. So you want to take element, and then add it with the next, and so forth.

var numbers = [1,2,3,4];
var sums = [];
for(var i in numbers) {
if((i + 1) in numbers) // So that we don’t add a non-existent fifth element.
sums.push(numbers[i] + numbers[i + 1]);
}

You’d probably assume (once you work through the “cleverness”) that you’d have [3, 5, 7] in sums, but sums will actually by empty. The reason for this is in the numbers[i + 1]. Javascript tries to be smart and do type coercion for you (0 == “0″) which is convenient alot of the time. However, here, i is “0″, “1″, etc., and _not_ 0, 1, 2. Javascript assumes you want strings over numbers, so it coerces the 1 to a string, instead of the i to a number, giving you “01″, “11″, “21″ as key values, which fails every time in our non-existent-fifth-element test.

It’s a contrived example here, but it will happen to everyone out there, and it’s very insidious since it looks so innocent and correct.

– Aaron