var jsonString = [
    {"ID":"1","name":"google","IP":"69.5.33.22","active":"true"},
    {"ID":"2","name":"bing","IP":"70.5.232.33","active":"false"}
];
var keyArray = Ext.Object.getKeys(Ext.JSON.decode(jsonString));
console.log(keyArray) // ["ID","name","IP","active"]