<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"></head><body><div class='triangle'></div>

    <script>
        var triangle = document.querySelector('.triangle');
        // 补全代码
        for(let i = 1; i <= 3; i++){
            for(let j = 0; j < i; j++){
                triangle.innerHTML += '*'
            }
            triangle.innerHTML += '<br>'
        }
    </script>
</body>

</html>