JSON.stringify打印循环引用的object对象

循环引用的obj数据，如果直接打印就是空，使用JSON转换打印会报错无法转换

Converting circular structure to JSON --> starting at object with constructor 'Socket'

1. 临时处理方案，覆写JSON的stringify方法再使用，引用数据会变成"client":"[Circular socket]"

    var handleCircular = function() {  
      var cache = [];
      var keyCache = []
      return function(key, value) {
          if (typeof value === 'object' && value !== null) {
              var index = cache.indexOf(value);
              if (index !== -1) {
                  return '[Circular ' + keyCache[index] + ']';
              }
              cache.push(value);
              keyCache.push(key || 'root');
          }
          return value;
      }
  }
 
  var tmp = JSON.stringify;  
  JSON.stringify = function(value, replacer, space) {  
      replacer = replacer || handleCircular();
      return tmp(value, replacer, space);
  }