Making Sprite-based Games with Canvas(jlongster.com)

    var now = Date.now();
    while(gameAge < now) {
       update();
       gameAge+=stepDuration;
    }
    render();
    //with usual caveats of resyncing if now-gameAge is too large

   bullets.push({ pos: [x, y],
                       dir: 'forward',
                       sprite: new 
  Sprite('img/sprites.png', [0, 39], [18, 8]) });

  var bullet = bullets[i];

        switch(bullet.dir) {
        case 'up': bullet.pos[1] -= bulletSpeed * dt; break;
        case 'down': bullet.pos[1] += bulletSpeed * dt; 
  break;
        default:
            bullet.pos[0] += bulletSpeed * dt;
        }

  bullets.push({ pos: [x, y],
                       dir: [0,10],
                       sprite: new 
  Sprite('img/sprites.png', [0, 39], [18, 8]) });

  bullet.pos[0] += bullet.dir[0] * dt;
  bullet.pos[1] += bullet.dir[1] * dt;