what is an "OWN" property in javascript

Question

Here i have a code which uses getter and setter to define and fetch a property value.I created an object using a object constructor.I passed the object in a for...in loop.Also used getOwnPropertyNames() method on the object.Here are the results

"fullName" property is accessible in for...in loop

"fullName" is not visible in getOwnPropertyNames() method.That means it is

not a own property

Here i have two basic question.What is an own property? If "fullName" is not a own property then what type of property it is ?

function Name(first, last) {
    this.first = first;
    this.last = last;
}

Name.prototype = {
    get fullName() {
        return this.first + " " + this.last;
    },

    set fullName(name) {
        var names = name.split(" ");
        this.first = names[0];
        this.last = names[1];
    }
};
var obj=new Name('al','zami');
for(var i in obj){
   console.log(i); // fullName is here
}
console.log(Object.getOwnPropertyNames(obj)); // fullName is not here

Answer 1

hasOwnProperty and getOwnPropertyNames refer to properties assigned directly to the object rather than only being accessible through the object's prototype chain. Either this.foo = or bar.foo = count as an own property, because you're assigning to the instance.

"Own properties" are defined in section 4.3.30 of the spec to be:

property that is directly contained by its object

vs an "inherited property," defined (4.3.31) as:

property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype

That is, an "own" property is on the instance, not the prototype.

The biggest impact is with prototypical classes (constructors with some methods and/or static properties). In classical OO terms, getOwnPropertyNames will skip class methods and anything that would have a static keyword.

If you take a look at section 8.12.1 of the spec, it indirectly excludes the prototype. In step #3, the runtime checks the object's own properties for the appropriate property name. However, in section 8.12.2 (referring to getProperty without the "own" qualifier), steps #3-4 describe checking the object's prototype if the property was not found on the object itself.

Answer 2

Object.getOwnPropertyNames returns the property names that are directly on the object. For instance if you add a new property to the object using obj.foo = 'bar'; then "foo" will be included in the object's own properties. fullName isn't a property directly on obj, but it is a property of this object:

{
    get fullName() {
        return this.first + " " + this.last;
    },

    set fullName(name) {
        var names = name.split(" ");
        this.first = names[0];
        this.last = names[1];
    }
}

which happens to be the internal prototype of obj:

Object.getOwnPropertyNames( Object.getPrototypeOf( obj ) ); // ["fullName"]

The for ...in loop iterates through all enumerable properties in the objects prototype chain, so it will include those properties.