FpsMeter.js

(function() {

var now;

now = Date.now || function() { 
    return (new Date).getTime();
};

function FpsMeter(opts) {
    opts || (opts = {});
    this.updateRate = opts.updateRate || 1000;
    this._tickCounter = 0;
    this._updateTimeoutId = null;
}

FpsMeter.prototype.start = function(callback) {
    var self = this;

    this._startTime = now();
    this._tickCounter = 0;
    this._updateTimeoutId = setTimeout(function() {
        if (self._tickCounter) {
            callback(Math.round(1000 / ((now() - self._startTime) / self._tickCounter)));
        }
        self.start(callback);
    }, this.updateRate);

    return this;
};

FpsMeter.prototype.stop = function() {
    clearTimeout(this._updateTimeoutId);

    return this;
};

FpsMeter.prototype.tick = function() {
    ++this._tickCounter;

    return this;
};

window.FpsMeter = FpsMeter;

}());