<!DOCTYPE html> <html> <body> <h1>JavaScript Set</h1> <p>向 Set 中添加新元素:</p> <p id="demo"></p> <script> // 创建新的 Set const letters = new Set(["a","b","c"]); // 添加新元素 letters.add("d"); letters.add("e"); // 显示 set.size document.getElementById("demo").innerHTML = letters.size; </script> </body> </html>