2d Canvas Game Library - Game Object

/*
WHAT IS IT
This is a game library object for helping to create games.

WHAT DOES IT DO
It will take the canvas and resize it to the window size even if the window is resized later by the user. It will also add a black background to the canvas. It will create a canvas object from the canvas in 2D context. It can also start a game loop and keep track of FPS.

Author: Zach Dyer
Website: ZachDyerDesign.com
Version: 0.1.0
*/

function Game(canvasID) {
    //Take out body spacing
    document.body.style.margin = 0;
    document.body.style.padding = 0;

    var canvasElement = document.getElementById(canvasID);

    //Canvas full screen with black background
    canvasElement.width = window.innerWidth;
    canvasElement.height = window.innerHeight;
    canvasElement.style.backgroundColor = "black";

    //Canvas size updates when window size changes
    window.onresize = function() {
        canvasElement.width = window.innerWidth;
        canvasElement.height = window.innerHeight;
    };

    this.canvas = canvasElement.getContext("2d");

    //Total frames per second
    this.fps = 0;
    var currentTime = new Number();
    var lastTime = new Number();

    //Can't use the this keyword in this.functions unless I save it in a variable
    var self = this;

    var interval;

    var getFPS = function(currentTime, lastTime) {
        var fps = 1000 / (currentTime - lastTime);
        return fps.toFixed();
    };

    var step = function(custom){
        currentTime = Date.now();
        self.fps = getFPS(currentTime, lastTime);
        lastTime = currentTime;
        custom();
    };

    this.loop = function (custom) {
    interval = setInterval(function(){
        step(custom);
        },17);
    };

}