Функция в canvas

function animate() { 
  var canvas = document.getElementById('canvas'); 
  var ctx = canvas.getContext('2d'); 
 
  function Square(x, y, w, h) { 
    this.x = Math.random() * 620; 
    this.y = y; 
    this.w = w; 
    this.h = h; 
    this.r = Math.round(Math.random() * 255); 
    this.g = Math.round(Math.random() * 255); 
    this.b = Math.round(Math.random() * 255); 
 
    this.rgba = "rgba(" + this.r + "," + this.g + "," + this.b + ",1)"; 
 
    this.draw = function() { 
      ctx.fillStyle = this.rgba; 
      ctx.fillRect(this.x, this.y, 20, 20); 
      this.update(); 
    } 
 
    this.update = function() { 
      this.y += Math.random() * 2; 
    } 
  } 
 
  var squares = []; 
 
  function draw() { 
    ctx.clearRect(0, 0, canvas.clientWidth, canvas.clientWidth); 
    for (i = 0; i < squares.length; i++) 
      squares[i].draw(); 
    update(); 
  } 
 
  function update() { 
    for (var i = 0; i < squares.length; i++) { 
      squares[i].update(); 
    } 
  } 
 
  setInterval(function() { 
 
    squares.push(new Square(0, 0, 20, 20)) 
  }, 1000); 
 
  setInterval(draw, 20); 
  var isCursorInSquares = function(x, y, squares) { 
    return x > squares.x && x < squares.x + squares.w && 
      y > squares.y && y < squares.y + squares.h; 
  } 
 
  canvas.onclick = function(e) { 
    var x = e.pageX; 
    y = e.pageY; 
 
    for (i in squares) 
      if (isCursorInSquares(x, y, squares[i])) 
        delete squares[i] 
  } 
} 
 
document.body.onload = animate;

<canvas id="canvas"></canvas>