Question

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?

Answer 1

The choice should be based on the which idiom is best understood.

An array is iterated using:

for (var i = 0; i < a.length; i++)    //do stuff with a[i] 

An object being used as an associative array is iterated using:

for (var key in o)   //do stuff with o[key] 

Unless you have earth shattering reasons, stick to the established pattern of usage.