javascript hasOwnProperty and prototype

Question

function Animal(name,numLegs){
this.name = name;
this.numLegs = numLegs}

Animal.prototype.sayName = function(){
console.log("Hi my name is " + this.name );}

var penguin = new Animal("Captain Cook", 2);
  penguin.sayName();
for (var prop in penguin){
console.log(prop);}
penguin.hasOwnProperty('sayName')

result:

name
numLegs
sayName
=> false

I dont know why hasOwnProperty return false?? can anyone explain?

Answer 1

When JavaScript is looking for a property, it first looks into the object itself. If it isn't there, it normally keeps walking up the prototype chain. hasOwnProperty exists to check only the object itself, explicitly not walking up the prototype chain. If you want to check if a property exists at all, checking everything in the prototype chain, use the in operator:

'sayName' in penguin  // => true