In loose mode, arrays are put in a fast path, thus heavily increasing performance. All other iterables will continue to work fine.
In loose mode an iterator's return method will not be called on abrupt completions caused by thrown errors.
Please see google/traceur-compiler#1773 and babel/babel#838 for more information.
This will apply the optimization shown below to all for-of loops by assuming that all loops are arrays.
Can be useful when you just want a for-of loop to represent a basic for loop over an array.
If a basic array is used, Babel will compile the for-of loop down to a regular for loop.