<!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>