Why defining properties in the prototype is considered an antipattern

Question

I often see this pattern to define javascript objects

function Person(name) {
    this.name = name;
}
Person.prototype.describe = function () {
    return \"         


        

Answer 1

In usual object-oriented languages, you have a definition of the class describing members, methods and the constructor.

In JS, the definition of the "class" (it is not really class like in other languages... sometimes the term pseudoclass is used) is the constructor itself. If your object is parametrised by name, it makes sense to write

function Person(name) {
    this.name = name;
}

i.e. the property name must be set in the constructor.

Of course, you can write

function Person(name) {
    this.name = name;
    this.describe = function() { ... };
}

and it will work as you expect.

However, in this case you are creating a separate instance of the method with every call of the constructor.

On the other hand, here:

Person.prototype.describe = function () {
    return "Person called "+this.name;
};

you only define the method once. All instances of Person will receive a pointer (called __proto__ and not accessible by programmer in most browsers) to Person.prototype. So if you call

var myPerson = new Person();
myPerson.describe();

it will work, because JS looks object members directly in the object, then in its prototype etc. all the way up to Object.prototype.

The point is that in the second case, only one instance of the function will exist. Which you will probably agree that is a better design. And even if you don't, it simply takes less memory.