test.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <script>
    function deepcopy(obj, map = new Map()) {
      // 把函数内变量转为参数使用
      if (typeof obj !== "object") {
        return;
      }
      let res = Array.isArray(obj) ? [] : {};

      if (map.get(obj)) {
        return map.get(obj);
      }
      map.set(obj, res);
      console.log("map", map);
      for (let key in obj) {
        if (obj.hasOwnProperty(key)) {
          if (typeof obj[key] === "object") {
            res[key] = deepcopy(obj[key], map); // 修改了此处,传入map
          } else {
            res[key] = obj[key];
          }
        }
      }
      return res;
    }

    var obj = {
      a: 1,
      b: 2,
      d: {
        a: 1,
      },
    };
    obj.c = obj;
    obj.d.b = obj.d;

    var deepcopyObj2 = deepcopy(obj);
    console.log(obj, deepcopyObj2);
  </script>

  <div></div>
  <style></style>
</html>