How does one manipulate a variable in a prototype?

Question

I'm new to Javascript and was wondering how a public variable in a prototype can be modified.

function Thing (val)
{
    this.x = val;

    this.addToX = function (valIn)
    {
        this.x += valIn;
    };
}

function ChildThing ()
{
    this.y = 55;
}

ChildThing.prototype = new Thing(10);

var frank = new ChildThing();

console.log("out1: " + frank.x);

frank.addToX(10);

console.log("out2: " + frank.x);

This code takes the value in the prototype x which is 10 and adds 10 to it in the addToX function. The new x value is stored in the top level object rather than replacing the current x value in the prototype.

Is there a way to overwrite the existing x in the prototype or am I using Javascript wrong?

Answer 1

That depends. What would be the point of altering x on the prototype? Generally you don't want to chang shared properties. But I imagine that there could be a use case (generating new id?).

As for the question: you can simply do:

this.addToX = function(valIn) {
    ChildThing.prototype.x += valIn;
};

Again I do not advice doing it.

EDIT You can make it without referencing the child by defining the prototype before setting it as a prototype, i.e.

var my_proto = new Thing(10);
ChildThing.prototype = my_proto;

and then

this.addToX = function(valIn) {
    my_proto.x += valIn;
};

Or you can even play with the singleton pattern.