Javascript Class/Static Variables

Question

I'm learning Javascript from Head First Javascript (Morrison).

In one example, the author declares a variable 'signature' to be a class property of a class called Blog by doing the following:

    Blog.prototype.signature = "by Blogger-Name";

Is there difference between the above and declaring it like how I have below?

    Blog.signature = "by Blogger-Name";

@vinayakj It isn't because my question is not about whether a function is referenced or copied. — rickyspanish
– rickyspanish, Commented May 1, 2015 at 6:14
@vinayakj I did go through that post and did not see my answer. if you still think my question is answered there, please be more specific. — rickyspanish
– rickyspanish, Commented May 1, 2015 at 6:40
@vinayakj While you have answered my question, the post you referred me to does not. I'll just leave it at that. — rickyspanish
– rickyspanish, Commented May 1, 2015 at 6:58

Ryan · Accepted Answer · 2015-05-01 06:00:01Z

3

All instances of Blog will have signature when you using .prototype. So when you instantiate var blog = new Blog. It will have a signature property.

If you were to just use Blog.signature = x Then when you create an object with new it wouldn't be there.

answered May 1, 2015 at 6:00

Ryan

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5 Comments

rickyspanish Over a year ago

Yes, but all instances can access the signature property through Blog.signature anyway. So what is the point of declaring properties through .prototype then?

Ryan Over a year ago

if you use both ways, then yes you can access it either way, but if you choose to use .prototype then you can think of it as a class member

rickyspanish Over a year ago

True, but why would you declare any variable that remains the same value for all instances of a class as an instance variable? You would only declare it as a class/static variable... I should probably change my question at this point to: Is there a scenario where you would be required to declare a variable using .prototype?

Ryan Over a year ago

I'd recommend asking a new question if you were planning on changing it.

rickyspanish Over a year ago

Yes, I think I'll do that.

6502 · Accepted Answer · 2015-05-01 06:22:18Z

Setting a property of a prototype member of a constructor makes the property visible (when reading) from all instances.

This can be done to emulate static members of C++ classes and is often done in Javascript form methods (because methods are indeed just regular data members, differently from C++).

One example where this could be used instead for "data" is keeping all instances of a certain "class" in a container:

function Widget(x0, y0, x1, y1, name) {
    this.x0 = x0;
    this.y0 = y0;
    this.x1 = x1;
    this.y1 = y1;
    this.name = name;
    this.all_instances.push(this);
}

Widget.prototype.all_instances = [];

var w1 = new Widget(10, 20, 30, 40, "First");
var w2 = new Widget(10, 20, 30, 40, "Second");

Note that however when writing to such a "static" property you're creating instead an instance property; push works because it's not writing to .all_instances but modifying its content (the array).

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