JavaScript判断对象是否为空对象的几种常见方法

var data = {};
var b = $.isEmptyObject(data); //true

var data = {};
var b = JSON.stringify(data) == "{}";
console.log(b); // true

var obj = {};
var b = function () {
  for (var key in obj) {
    return false;
  }
  return true;
};
console.log(b()); //true

var data = {};
var b = $.isEmptyObject(data);
console.log(b); //true

var data = {};
var arr = Object.getOwnPropertyNames(data);
console.log(arr.length == 0); // true

var data = {};
var arr = Object.keys(data);
console.log(arr.length == 0); // true

JSON.stringify(value[, replacer[, space]])

const data = {
  a: "aaa",
  b: undefined,
  c: Symbol("dd"),
  fn: function () {
    return true;
  },
};
JSON.stringify(data); // 输出：？
// "{"a":"aaa"}"

JSON.stringify([
  "aaa",
  undefined,
  function aa() {
    return true;
  },
  Symbol("dd"),
]); // 输出：？
// "["aaa",null,null,null]"

JSON.stringify(function a() {
  console.log("a");
});
// undefined
JSON.stringify(undefined);
// undefined
JSON.stringify(Symbol("dd"));
// undefined

const data = {
  a: "aaa",
  b: undefined,
  c: Symbol("dd"),
  fn: function () {
    return true;
  },
  d: "ddd",
};
JSON.stringify(data); // 输出：？
// "{"a":"aaa","d":"ddd"}"
JSON.stringify([
  "aaa",
  undefined,
  function aa() {
    return true;
  },
  Symbol("dd"),
  "eee",
]); // 输出：？
// "["aaa",null,null,null,"eee"]"

JSON.stringify({
    say: "hello JSON.stringify",
    toJSON: function() {
      return "today i learn";
    }
  })
// "today i learn"

JSON.stringify({ now: new Date() });
// "{"now":"2019-12-08T07:42:11.973Z"}"

JSON.stringify(NaN)
// "null"
JSON.stringify(null)
// "null"
JSON.stringify(Infinity)
// "null"

JSON.stringify([new Number(1), new String("false"), new Boolean(false)]);
// "[1,"false",false]"

// 不可枚举的属性默认会被忽略：
JSON.stringify( 
    Object.create(
        null, 
        { 
            x: { value: 'json', enumerable: false }, 
            y: { value: 'stringify', enumerable: true } 
        }
    )
);
// "{"y":"stringify"}"

// 对包含循环引用的对象（对象之间相互引用，形成无限循环）执行此方法，会抛出错误。 
const obj = {
  name: "loopObj"
};
const loopObj = {
  obj
};
// 对象之间形成循环引用，形成闭环
obj.loopObj = loopObj;
// 封装一个深拷贝的函数
function deepClone(obj) {
  return JSON.parse(JSON.stringify(obj));
}
// 执行深拷贝，抛出错误
deepClone(obj)
/**
 VM44:9 Uncaught TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'loopObj' -> object with constructor 'Object'
    --- property 'obj' closes the circle
    at JSON.stringify (<anonymous>)
    at deepClone (<anonymous>:9:26)
    at <anonymous>:11:13
 */