Rule Definition
Since each iteration through the loop results in a property lookup either on the instance or on a prototype, the for-in loop has considerably more overhead per iteration and is therefore slower than the other loops. For the same number of loop iterations, a for-in loop can end up as much as seven times slower than the other loop types. For this reason, it's recommended to avoid the for-in loop unless your intent is to iterate over an unknown number of object properties.

Remediation
Replace the for-in with regular for with index

Violation Code Sample

function printArray(arr) { 
    for (var key in arr) { 
        print(arr[key]); 
    } 
}

Fixed Code Sample

function printArray(arr) { 
    var l = arr.length; 
    for (var i = 0; i < l; i++) { 
        print(arr[i]); 
    } 
}

CISQ OMG