SOURCE

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");
var x=canvas.width/2;
var y=canvas.height-30;
var dx=2;
var dy=-2;
 function drawBall(){
ctx.beginPath();
    ctx.arc(x,y,20,0,Math.PI*2,false);
    ctx.fillStyle="#FF0000";
    ctx.fill();
    ctx.closePath();
   }
function draw(){
drawBall();
x+=dx;
y+=dy;
}
setInterval(draw,10)

<canvas id="myCanvas" width="480" height="320"></canvas>