import QtQuick 2.3

DropArea{
    keys: ["A","B","C"]
    Component.onCompleted: {
        console.log(JSON.stringify(keys));
    }
}

Output:

{"0":"A","1":"B","2":"C"}

It seems that QML creates number string keys in the Array object. This manner is very slow for huge data.
The output should be:

[”A”,”B”,”C”]