How does hasOwnProperty work on properties inherited from the prototype?

Question

I have been reading through Mozilla Developer Network lately on JavaScript inheritance model. I am highly confused on one point. Here's the code from MDN:

function Graph() {
  this.vertices = [];
  this.edges = [];
}
Graph.prototype = {
  addVertex: function(v) {
  this.vertices.push(v);
}
};
var g = new Graph();
console.log(g.hasOwnProperty('vertices'));// true
console.log(g.hasOwnProperty('addVertex'));// false
console.log(g.__proto__.hasOwnProperty('addVertex'));// true

What I don't understand is that why does g.hasOwnProperty('addVertex') yields false since addVertex is a property of g although it's defined in Graph's prototype but still it is part of Graph.

Answer 1

Because hasOwnProperty specifically says it return false on inherited properties

MDN:

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as own (not inherited) property.

As for your second question - it depends on exactly how you have an object inherit from Graph. In the ES5 way, I would do this:

var InheritedFromGraph = function() {
  Graph.call(this);
}

InheritedFromGraph.prototype = Graph.prototype;

and then, yes, InheritedGraph will get the properties verticies and edge that Graph defined in it's constructor.