自定义JSON循环结构处理函数导致输出截断的原因解析

问题重现 #

为解决JSON转换循环结构报错，编写了以下处理函数：

circular = () => { //fix circular stuff for json.stringify
  seen = new WeakSet();
  return (key, value) => {
    if (typeof value === 'object' && value !== null) {
      if (seen.has(value)) {
        return;
      }
      seen.add(value);
    }
    return value;
  };
};

在以下场景中出现输出"截断"问题：

var gameon = 1;
var fighter1 = {"userid":"97","username":"john","items":{},"ailments":{}};
var fighter2 = {"userid":"91","username":"james","items":{},"ailments":{}};
var resume = 30;

all = {"gameon":gameon,"fighter1":fighter1,"fighter2":fighter2,"resume":30,"inturn":fighter1,"outturn":fighter2};
    
console.log(JSON.stringify(all,circular()));

执行后输出仅到resume字段就结束，后续的inturn和outturn字段丢失：

{"gameon":1,"fighter1":{"userid":"97","username":"john","items":{},"ailments":{}},"fighter2":{"userid":"91","username":"james","items":{},"ailments":{}},"resume":30}

（注：你提供的输出末尾多了一个多余引号，应为输入失误）

原因分析 #

1. 重复引用返回undefined导致键值对被忽略：
   当JSON.stringify遍历到inturn: fighter1时，fighter1对象已经在处理fighter1字段时被加入了seen WeakSet，此时处理函数返回undefined。而JSON.stringify的规则是：如果replacer函数返回undefined，对应的键值对会被直接跳过，不会出现在最终的JSON字符串中。同理outturn: fighter2也会被跳过，看起来像是输出被"截断"。

2. 全局变量seen的潜在污染：
   函数中的seen没有用let/const声明，属于隐式全局变量。如果多次调用circular()，会导致seen被重复覆盖，可能引发其他场景下的异常。

修复方案 #

修改处理函数，针对重复引用返回可序列化的标识，同时将seen改为闭包局部变量：

const circular = () => { 
  const seen = new WeakSet(); // 改为局部变量，避免全局污染
  return (key, value) => {
    if (typeof value === 'object' && value !== null) {
      if (seen.has(value)) {
        // 返回自定义标识，而非undefined，保留键值对
        return `[Circular Reference: ${value.userid || key}]`;
        // 也可以直接返回对象的唯一标识，比如value.userid
        // return value.userid;
      }
      seen.add(value);
    }
    return value;
  };
};

修改后，再执行原测试代码，输出会保留inturn和outturn字段，显示为自定义的循环引用标识，不会出现"截断"：

{"gameon":1,"fighter1":{"userid":"97","username":"john","items":{},"ailments":{}},"fighter2":{"userid":"91","username":"james","items":{},"ailments":{}},"resume":30,"inturn":"[Circular Reference: 97]","outturn":"[Circular Reference: 91]"}

内容的提问来源于stack exchange，提问作者Cain Nuke