function Person(name, age, height) {
this.name = name,
this.age = age,
this.height = height
}
var john = new Person("John", 24, 175);
What I read should be avoided:
john.__proto__.fact = "I am a humanly human!"
What should be done instead:
Person.prototype.fact = "I am a humanly human!"
If you want to add to the prototype of the object john refers to, doing so through john.__proto__ gives the impression to the reader of the code that this change is specific to john somehow, which it isn't; it affects all objects that use that prototype (current and future). Using Person.prototype doesn't give that impression, it correctly indicates what you're doing.
Separately, note that not all objects (not even all objects with prototypes) have __proto__, because __proto__ is inherited from Object.prototype, but not all objects inherit from Object.prototype. For that reason, in new code, it's best to avoid __proto__ entirely; use Object.getPrototypeOf instead if you need to get the prototype of an object: Object.getPrototypeOf(john).
In a comment you've answered the question of why you'd want to do this with:
To add methods such that they are not present in every Person object you create.
...but that's exactly what adding to john.__proto__ would do: Add the method to every Person object (indirectly through the prototype), current and future, exactly like adding to Person.prototype — because those (and Object.getPrototypeOf(john)) are all just different ways of getting to the same object:
function Person(name) {
this.name = name;
}
var john = new Person("John");
console.log(john.__proto__ === Person.prototype); // true
console.log(Object.getPrototypeOf(john) === Person.prototype); // true
var paul = new Person("Paul");
john.__proto__.someNewMethod = function() {
console.log(this.name);
};
john.someNewMethod(); // "John"
paul.someNewMethod(); // "Paul"If you want to add a method just to the object john refers to, don't use __proto__ or Person.prototype at all: Just do:
john.niftyMethod = function() { /*....*/ };
