platformer javascript

			//pt1

			//if you don't have a canvas, this adds it
            if(document.getElementsByTagName('canvas').length == 0) {
                var canvas = "<canvas id='canvas' width='800' height='500'></canvas>"
                document.body.innerHTML += canvas;
            }
            
            //variable declaration
            var keys = [];
            var ctx = document.getElementById('canvas').getContext('2d');
            var level = [];
            
            //events
            document.body.addEventListener("keydown", function(e) {
                keys[e.keyCode] = true;
            });
            document.body.addEventListener("keyup", function(e) {
                keys[e.keyCode] = false;
            });
            
            //make the level and player
            var player = {
                x: 0,
                y: 0,
                yv: 0,
                xv: 0,
                slope: 0,
                width: 25,
                height: 25,
                color: '#FCA738'
            };
            
            var ground = {
                x: 0,
                y: 470,
                width: 800,
                height: 30,
                color: '#155261'
            };
            
            var ceiling = {
                x: 0,
                y: 0,
                width: 800,
                height: 30,
                color: '#155261'
            };
            
            
            var leftWall = {
                x: 0,
                y: 0,
                width: 30,
                height: 600,
                color: '#155261'
            };
            
            var rightWall = {
                x: 770,
                y: 0,
                width: 30,
                height: 600,
                color: '#155261'
            };
            
            var ceilingBlock = {
                x: 100,
                y: 400,
                width: 50,
                height: 20,
                color: '#155261'
            }
            
            //this pushes all of the static objects into the level
            level.push(ground, leftWall, rightWall, ceilingBlock, ceiling);
            
            //start the engine
            window.onload = start;
            
            //this function is called at the start
            function start() {
                player.x = 50;
                player.y = 400;
                update();
            }
            
            //this function is called every frame
            function update() {
                requestAnimationFrame(update);
                drawPlayer();
                drawLvl();
                //this function takes in the following:
                //the player | the level | the player speed | the player gravity //the player friction | the player jump height
                physics(player, level, 1.5, 0.7, 0.9, 9);
            }
            
            //this function draws the player
            function drawPlayer() {
                ctx.clearRect(0, 0, 800, 500);
                ctx.fillStyle = player.color;
                ctx.fillRect(player.x, player.y, player.width, player.height);
            }
            
            //this function draws the level
            function drawLvl() {
                for (var i = 0; i < level.length; i++) {
                    ctx.fillStyle = level[i].color;
                    ctx.fillRect(level[i].x, level[i].y, level[i].width, level[i].height);
                }
            }

            //this function handles the platformer physics
            function physics(p1, lvl, speed, gravity, friction, jumpheight) {
                //gravity
                p1.yv += gravity;
                p1.y += p1.yv;
                
                //y collision
                for(var i = 0; i < lvl.length; i++) {
                    if(collisionBetween(p1, lvl[i])) {
                        p1.y += -p1.yv;
                        if(keys[38]) {
                            p1.yv = -jumpheight;
                        } else {
                           p1.yv = 0; 
                        }
                    }
                }