Question

I usually script/program using python but have recently begun programming with JavaScript and have run into some problems while working with arrays.

In python, when I create an array and use for x in y I get this:

myarray = [5,4,3,2,1]
for x in myarray:
    print x

and I get the expected output of:

5
4
3
..n

But my problem is that when using Javascript I get a different and completely unexpected (to me) result:

var world = [5,4,3,2,1]
for (var num in world) {
    alert(num);
}

and I get the result:

0
1
2
..n

How can I get JavaScript to output num as the value in the array like python and why is this happening?

Answer 1

JavaScript and Python are different, and you do things in different ways between them.

In JavaScript, you really should (almost) always iterate over an array with a numeric index:

for (var i = 0; i < array.length; ++i)
  alert(array[i]);

The “for … in” construct in JavaScript gives you the keys of the object, not the values. It’s tricky to use on an array because it operates on the array as an object, treating it no differently than any other sort of object. Thus, if the array object has additional properties — which is completely “legal” and not uncommon — your loop will pick those up in addition to the indexes of the “normal” array contents.