[JS] Why to use hasOwnProperty?

hasOwnProperty.js

// Poisoning Object.prototype
Object.prototype.bar = 1;
var foo = {goo: undefined};

console.log(foo.bar); // 1
console.log('bar' in foo); // true

console.log(foo.hasOwnProperty('bar')); // false
console.log(foo.hasOwnProperty('goo')); // true

//not safe without hasOwnProperty
for(var index in foo) console.log(index, foo[index]);

//safe
console.log(Object.keys(foo));