why this function is in all of the object even though i only mentioned it in one

Question

I've added a method calcAge to jonas, but why is it working for matilda?

const Person = function (firstName, birthYear) {
  this.firstName = firstName;
  this.birthYear = birthYear;
};

const jonas = new Person('jonas', 1991);
const matilda = new Person('Matilda', 2017);

jonas.__proto__.calcAge = function () {
  console.log(2037 - this.birthYear);
};
jonas.calcAge(); // 46
matilda.calcAge(); // 20

`__proto__` is a property which accesses the object's prototype. When you add a method to it makes that method available to all other instances of the prototype. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/proto — evolutionxbox, Jul 08 '23 at 13:11
i got it if you really wanna put property in only one object then it dosent make sense to put that in prototype i got it — Bilal Zahid Shah, Jul 08 '23 at 13:13
i am new in javascript and still learing it Objects in js work quite diffrent then other langs — Bilal Zahid Shah, Jul 08 '23 at 13:14
If you only want it on one instance, remove `__proto__`? `jonas.calcAge = function ...` — evolutionxbox, Jul 08 '23 at 13:16

score 0 · Answer 1 · answered Jul 08 '23 at 15:56

You added the method to proto which all objects inherit from. so jonas.proto and matilda.proto reference the same thing. You can think of it like a family tree.

example: carl.mom and jane.mom if carl and jane are brother and sister. mom is the same person.

hope this makes sense.

why this function is in all of the object even though i only mentioned it in one

1 Answers1