js判断空对象的方法
判断一个js对象是否是空对象isEmptyObject
author: @TiffanysBear

方法一：使用for...in遍历

var isEmptyObject = function () {
    for (var i in this) {
        return false;
    }
    return true;
}
// 尽量不要使用object.prototype直接进行修改
// 否则会为继承时生成的对象新增不必要的可枚举属性
// 同时可被for-in枚举到
Object.defineProperty(Object.prototype, 'isEmptyObject', {
    writable: false,
    configurable: false,
    enumerable: false,
    value: isEmptyObject
});

方法二：使用JSON.stringify方法

var isEmptyObject = function () {
    return JSON.stringify(obj) === '{}';
}

Object.defineProperty(Object.prototype, 'isEmptyObject', {
    writable: false,
    configurable: false,
    enumerable: false,
    value: isEmptyObject
});

方法三：使用ES6的Object.keys

var isEmptyObject = function () {
    return Object.keys(a).length === 0;
}

Object.defineProperty(Object.prototype, 'isEmptyObject', {
    writable: false,
    configurable: false,
    enumerable: false,
    value: isEmptyObject
});

如果不支持Object.keys，采用如下的polyfill：

if (!Object.keys) {
  Object.keys = (function () {
    var hasOwnProperty = Object.prototype.hasOwnProperty,
        hasDontEnumBug = !({toString: null}).propertyIsEnumerable('toString'),
        dontEnums = [
          'toString',
          'toLocaleString',
          'valueOf',
          'hasOwnProperty',
          'isPrototypeOf',
          'propertyIsEnumerable',
          'constructor'
        ],
        dontEnumsLength = dontEnums.length;

    return function (obj) {
      if (typeof obj !== 'object' && typeof obj !== 'function' || obj === null) {
          throw new TypeError('Object.keys called on non-object');
      }

      var result = [];

      for (var prop in obj) {
        if (hasOwnProperty.call(obj, prop)) result.push(prop);
      }

      if (hasDontEnumBug) {
        for (var i=0; i < dontEnumsLength; i++) {
          if (hasOwnProperty.call(obj, dontEnums[i])) result.push(dontEnums[i]);
        }
      }
      return result;
    }
  })()
};