requestAnimationFrame and time since last frame (seconds)

loop.js

var Loop = function (step) {
    var self = this;
    var lastFrameTimestamp = null;
    var nextStep = function (timestamp) {
        if (lastFrameTimestamp === null) {
            lastFrameTimestamp = timestamp;
        }
        var tslf = timestamp - lastFrameTimestamp;
        step(tslf);
        lastFrameTimestamp = timestamp;
        self.requestID = window.requestAnimationFrame(nextStep);
    };
    
    self.stop = function() {
        window.cancelAnimationFrame(self.requestID);
    }

    nextStep();
};