Javascript Object instantiated inside its declaration

Question

Can someone explain how this works. An object checks to see if it has been instantiated, and if it hasn't, then instantiates itself. It reminds me of a singleton, but I am not sure if I understand this code correctly.

var circularBuffer = function (size) {

    if (this instanceof  circularBuffer) {
        this.size = size;
        this.clear();
    } else {
        return new circularBuffer(size);
    }

};

Answer 1

This pattern makes sure that whether you call circularBuffer with new or not, you still get a new instance.

So both of the following result in a new circularBuffer instance being assigned to cb:

var cb = new circularBuffer(100);

var cb = circularBuffer(100);

In the first case, this is a circularBuffer instance so it follows the if path. In the second case, this is window so the method follows the else path and will re-call itself using new instead.