The risk is that `for (x in y)` loops over the properties of the object, while `y.forEach` is array-only and loops over the array elements. A difference could occur if you add a property to an array,

````
var x = [1,2,3];
x.key = 'value';
````

then the two iteration methods would differ.

The risk is small, but to avoid this and for consistency, it's better to do `forEach` on arrays.