Why extend a function with an empty object?

Question

I was looking at the Rebound javascript code and saw that the SpringSystem function is being extended with an empty object.

var SpringSystem = rebound.SpringSystem = function SpringSystem() {
    this._springRegistry = {};
    this._activeSprings = [];
    this._listeners = [];
    this._idleSpringIndices = [];
    this._boundFrameCallback = bind(this._frameCallback, this);
};

extend(SpringSystem, {});

Where extend looks like:

function extend(target, source) {
    for (var key in source) {
        if (source.hasOwnProperty(key)) {
            target[key] = source[key];
        }
    }
}

What does the first extend achieve by extending an empty object? It's my understanding that an empty object has no hasOwnProperty(key) for anything.

What am I missing?

Answer 1

You are missing nothing, the code serves no purpose. It looks like a leftover from a class template, notice that other classes and all prototypes use this pattern.

That's also why it has been removed in 2014.