console 命令行工具 X clear

console

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var raf;
var running = false;

var ball = {
  x: 100,
  y: 100,
  vx: 5,
  vy: 1,
  radius: 25,
  color: 'blue',
  draw: function() {
    ctx.beginPath();
    ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2, true);
    ctx.closePath();
    ctx.fillStyle = this.color;
    ctx.fill();
  }
};

function clear() {
  ctx.fillStyle = 'rgba(255,255,255,0.3)';
  ctx.fillRect(0,0,canvas.width,canvas.height);
  ctx.clearRect(0,0,canvas.width,canvas.height);
}

function draw() {
  clear();
  ball.draw();
  ball.x += ball.vx;
  ball.y += ball.vy;

  if (ball.y + ball.vy > canvas.height || ball.y + ball.vy < 0) {
    ball.vy = -ball.vy;
  }
  if (ball.x + ball.vx > canvas.width || ball.x + ball.vx < 0) {
    ball.vx = -ball.vx;
  }

  raf = window.requestAnimationFrame(draw);
}

canvas.addEventListener('mousemove', function(e){
  if (!running) {
    clear();
    ball.x = e.clientX;
    ball.y = e.clientY;
    ball.draw();
  }
});

canvas.addEventListener('click',function(e){
  if (!running) {
    raf = window.requestAnimationFrame(draw);
    running = true;
  }
});

canvas.addEventListener('mouseout', function(e){
  window.cancelAnimationFrame(raf);
  running = false;
});

ball.draw();

<html>
 <body>
   <canvas id="canvas" style="border: 1px solid" width="600" height="300"></canvas>
 </body>
</html>

*{
    margin:0;
    padding:0;
}