If you have extended Array prototype with functions like this :

Array.prototype.inArray = function(){
    console.log('inArray function');
}

The result of your ‘for’ will contain the properties and new functions.

var values = ['value1', 'value2'];
console.log(testArray);

// => ["value1", "value2", inArray: function]

for(var i in values){
    console.log(i + ' : ' + values[i]);
}

// =>
// 0 : value1
// 1 : value2
// inArray : function (){
//   console.log('inArray function');
// }

To fix this issue :

Case 1 with hasOwnProperty

for(var i in values){
    if (values.hasOwnProperty(i)) {
        console.log(i + ' : ' + values[i]);
    }
}

// =>
// 0 : value1
// 1 : value2

Case 2 with forEach

function logArrayElements(element, index, array) {
    console.log("a[" + index + "] = " + element);
}
values.forEach(logArrayElements);

// =>
// 0 : value1
// 1 : value2

Run on JsFiddle