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              <div id="viewport"></div>

            
          
!
            
              #viewport {
  overflow: hidden;
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;
  height: 300px;
  width: 400px;
  border: 1px solid #666;
}

.ball {
  position: absolute;
  height: 20px;
  width: 20px;
  border-radius: 10px;
  background-color: #7777ff;
}
            
          
!
            
              const viewportHeight = 300;
const viewportWidth = 400;
const threshold = 15;

class Ball {
  constructor() {
    this.$el = $('<div class="ball"></div>');
    
    this.cx = 200;
    this.cy = 150;
    
    this.vx = this.vy = 0;
    
    this.ax = this.ay = 0;
    
    this.radius = 10;
    
    this.renderAt(this.cx, this.cy);
    
    this.behavior = moveBehavior;
    
    this.$el
      .on('mouseenter', () => this.behavior = effectBehavior)
      .on('mouseleave', () => this.behavior = moveBehavior);
  }
  
  getColor() {
    let m = this.$el.css('background-color').match(/^rgb\((\d+),\s*(\d+),\s*(\d+)\)$/);
    return {
      r: m[1],
      g: m[2],
      b: m[3]
    }
  }
  
  setColor(r, g, b) {
    this.$el.css('background-color', `rgb(${r}, ${g}, ${b})`);
  }
  
  accelerate(code) {
    this.ax = this.ay = 0;
    
    switch (code) {
      case 37: // left
        this.ax = -0.1;
        break;
      case 38: // up
        this.ay = -0.1;
        break;
      case 39: // right
        this.ax = 0.1;
        break;
      case 40: // down
        this.ay = 0.1;
        break;
      default:
        // do nothing
    }
  }
  
  renderAt(x, y) {
    this.$el.css('transform', `translate3d(${x - this.radius}px, ${y - this.radius}px, 0)`);
  }
  
  update() {
    this.behavior(this);
  }
}

function moveBehavior(b) {
  if (b.cx - b.radius < 0) {
    b.cx = b.radius;
    b.vx *= -1;
  } else if (b.cx + b.radius > viewportWidth) {
    b.cx = viewportWidth - b.radius;
    b.vx *= -1;
  }
    
  if (b.cy - b.radius < 0) {
    b.cy = b.radius;
    b.vy *= -1;
  } else if (b.cy + b.radius > viewportHeight) {
    b.cy = viewportHeight - b.radius;
    b.vy *= -1;
  }
 
  b.vx += b.ax;
  b.vy += b.ay;
  b.cx += b.vx;
  b.cy += b.vy;
    
  b.renderAt(b.cx, b.cy);
  
  if (b.vx ** 2 + b.vy ** 2 > threshold ** 2) {
    b.behavior = leaveBehavior;
  }
}

function effectBehavior(b) {
  let {r: red, g: green, b: blue} = b.getColor();
  b.setColor((red + 1) % 255, green, blue);
}

function leaveBehavior(b) {
  b.vx += b.ax;
  b.vy += b.ay;
  b.cx += b.vx;
  b.cy += b.vy;
    
  b.renderAt(b.cx, b.cy);
}

$(() => {
  let b = new Ball();
  $('#viewport').append(b.$el);
  
  $(window)
    .on('keydown', (e) => b.accelerate(e.which))
    .on('keyup', () => b.accelerate());
  
  requestAnimationFrame(animate);
  
  function animate(timestamp) {
    b.update(timestamp);
    requestAnimationFrame(animate);
  }
});
            
          
!
999px
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