JavaScript for…in vs for

#short
#javascript
Do you think there is a big difference in for…in and for loops? What kind of “for” do you prefer to use and why?
Let’s say we have an array of associative arrays:
var myArray = [{‘key’: ‘value’}, {‘key’: ‘value1’}];

So we can iterate:
for (var i = 0; i myArray.length; i++)

And:
for (var i in myArray)

I don’t see a big difference. Are there any performance issues?