Should hasOwnProperty still be used with for..in statements

Question

There are a lot of blogs saying that a hasOwnProperty check should be used whenever the for..in loop is used, but I don\'t understand why that\'s t

Answer 1

Without hasOwnProperty you don't know whether the property is a native property of your object or inherited from it's prototype.

Your modified fiddle

var obj1 = {a:"10",b:"20"};

Object.prototype.c = "prototype:30";

var i;
for(i in obj1) {
    document.getElementById("div1").innerHTML += obj1[i]+" ";
}
// result 10 20 prototype:30

for(i in obj1) {
    if(obj1.hasOwnProperty(i)) {
        document.getElementById("div2").innerHTML += obj1[i] +" ";
    }          
}
// result 10 20
​

In this case obj1 inherits the property c from it's Prototype Object and you would erroneously list it in your first loop.