<canvas width="300" height="300"></canvas>
<script>

    var canvas = document.querySelector('canvas'),
        ctx = canvas.getContext('2d');

    var Bullet = function (x, y, vx, vy) {
        this.x = x;
        this.y = y;
        this.vx = vx;
        this.vy = vy;
    };

    Bullet.prototype.tick = function (dt) {
        this.x += dt * this.vx;
        this.y += dt * this.vy;
    }

    Bullet.prototype.draw = function (ctx) {
        ctx.beginPath();
        ctx.arc(this.x, this.y, 10, 0, 2*Math.PI);
        ctx.stroke();
    }

    var bullets = [];
    bullets.push(new Bullet(0, 150, 200, 40));
    bullets.push(new Bullet(0, 150, 200, 0));
    bullets.push(new Bullet(0, 150, 200, -40));

    window.setInterval(function () {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        bullets.forEach(function (b) {
            b.tick(1 / 60);
            b.draw(ctx);
        });
    }, 1000 / 60);

</script>