Strange co-incidence, I just wrote one earlier today! I don't know what exactly your requirements are but this might be of use.

It presents an interface like an Array of unlimited length, but ‘forgets’ old items:

// Circular buffer storage. Externally-apparent 'length' increases indefinitely

// while any items with indexes below length-n will be forgotten (undefined

// will be returned if you try to get them, trying to set is an exception).

// n represents the initial length of the array, not a maximum

function CircularBuffer(n) {

    this._array= new Array(n);

    this.length= 0;

}

CircularBuffer.prototype.toString= function() {

    return '[object CircularBuffer('+this._array.length+') length '+this.length+']';

};

CircularBuffer.prototype.get= function(i) {

    if (i<0 || i<this.length-this._array.length)

        return undefined;

    return this._array[i%this._array.length];

};

CircularBuffer.prototype.set= function(i, v) {

    if (i<0 || i<this.length-this._array.length)

        throw CircularBuffer.IndexError;

    while (i>this.length) {

        this._array[this.length%this._array.length]= undefined;

        this.length++;

    }

    this._array[i%this._array.length]= v;

    if (i==this.length)

        this.length++;

};

CircularBuffer.IndexError= {};