思路:

  1. 使用字符串 repeat() 方法重复 i 次字符 '*'
  2. 修改某个元素的 HTML 内容使用 .innerHTML
<!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
            let character = "*";
            let ans = "";
            for (let i = 1; i <= 3; i++) {
                ans = ans + character.repeat(i) + "<br>";
            }
            triangle.innerHTML = ans;
        </script>
    </body>
</html>