Question

Is this code,

function Person() {
    function  myMethod() {
        alert ('hello');
    }
    this.method = myMethod;
}

equivalent to:

function Person() {    }
Person.prototype.method2  = function() {
    alert ('hello');
};

If yes, which method definition should I use and why?

Answer 1

They are functionally equivalent in your simple example, but behind the scenes work very differently. The prototype property on a function is really the “prototype template”. It says “whenever an object is made and I am used as the object’s constructor, give them this object as their prototype”.

So all Persons created in your second example share the same copy of the method2 method.

In the first example, each time the interpreter encounters the function keyword, then it creates a new function object. So in the first example, each instance of Person has their own copy of the myMethod method. The vast majority of the time this doesn’t matter. But this first approach uses more memory, and sometimes that does matter.

They are not functionally equivalent in more interesting cases. In the first example, myMethod can access local variables defined in Person, but the second example cannot, as one difference.