Why does JavaScript prototype method output differently for the same property?

Question

Can someone explain to me like I lack basic brain function why fluffy.age outputs 3 while muffin.age outputs 4 ? Is it simply because fluffy is defined prior to the age change?

I'm extremely new to coding but already have a decent grasp on everything else JavaScript; However, the prototype idea still makes little sense to me.

 function Cat(name, color, meow) { this.name = name; this.color = color; } Cat.prototype.age = 3; var fluffy = new Cat("Fluffy", "White"); console.log(fluffy.age); Cat.prototype = { age: 4 }; console.log(fluffy.age); var muffin = new Cat("Muffin", "Brown"); console.log(muffin.age); 

Answer 1

You are updating the prototype after you created an instance from that. So that instance does not contain updated prototype properties.

Answer 2

 function Cat(name, color, meow) { this.name = name; this.color = color; } Cat.prototype.age = 3; var fluffy = new Cat("Fluffy", "White"); console.log('Before update', fluffy.age); fluffy.age = 4; console.log('After update', fluffy.age); var muffin = new Cat("Muffin", "Brown"); console.log('New object', muffin.age); 

When you create a new instance from a class and if you want to change the values in that instance, you need to use that instance not the original class