Question

var Person = function(name){
    this.name = name;
};
console.log("1 " + typeof(Person.prototype)) //"object" - prototype is a property of a first class functional object

Person.prototype.sayhi = function(){console.log("hi")};

var john = new Person();

console.log("2 "+typeof(Person.sayhi)) // "undefined"
console.log("3 "+typeof(john.sayhi))// "function"

I am trying to get a better understanding of javascript prototype.

I wonder why case 2 returns undefined, while case 3 returns “object” as it should.

I read up on other posts but can’t seem to find the answer. Thanks.

Answer 1

Functions attached to the prototype (Person.prototype) are not accessible from a constructor (Person), that is Person.sayhi is not trying to access the prototype at all.

When you call a constructor (say var p = new Person()), Person.prototype is attached to the prototype chain of the created object (p), that’s why you can call p.sayhi(). However, sayhi is never attached to the constructor