Why does Object.assign add to the prototype's constructor?

Question

I am learning composition in Javascript and was wondering why Object.assign() in my code is adding methods to the prototype's constructor rather than the object's constructor. I will link the code below. Basically, the code is adding two methods (eat and walk) to the Person object by using Object.assign(). But, as the image below shows, the console is displaying the methods as belonging to the Person.prototype's constructor rather than the Person object itself. I must be overthinking it. Any help is appreciated. Thanks!

Code

const canEat = {
    eat() {
        this.hunger--;
        console.log('eating');
    }
};

const canWalk = {
    walk: function() {
        console.log('walking');
    }
};

const canSwim = {
    swim: function() {
        console.log("swim");
    }
};

function Person() {

}

Object.assign(Person,canEat,canWalk);
const person = new Person();
console.log(person);

antonku · Accepted Answer · 2019-07-20T06:51:17.383

When Object.assign is evaluated:

Object.assign(Person, canEat, canWalk);

eat and walk properties are assigned to Person function.

When the instance of Person is created:

const person = new Person();

Then we have a new object which has a constructor property pointed to the Person function. However the new object has neither eat nor walk properties (those belong to Person [constructor] function).

If you want to make eat and walk properties available for instances of Person, you should assign those methods to Person.prototype instead:

Object.assign(Person.prototype, canEat, canWalk);

Because when an object is created with new operator, it inherits not from the constructor function Person, but from constructor function prototype Person.prototype.

score 1 · Answer 2 · answered Jul 20 '19 at 06:40

You're not examining the Person object. It kind of looks like you are if you're not familiar with the console.log output, because the first line says Person, but you're logging an instance of Person, not Person itself.

The prototype of the instance you're logging is Person.prototype, and the Person.prototype object has a constructor property referring to the Person function. When you see eat and walk listed as properties under constructor, that means they're properties of the Person function, as they should be.

Why does Object.assign add to the prototype's constructor?

2 Answers2