JS Testing Properties 判断属性是否在对象里的方法
作者：流畅的心情
Testing Properties
To check whether an object has a property with a given name. You can do this with the in operator, with the hasOwnProperty() and propertyIsEnumerable() methods,
在JS中判断一个对象是否包含某个属性，可以使用 in，hasOwnProperty() and propertyIsEnumerable()
or simply by querying the property.
或者直接使用查询属性。
in--It returns true if the object has an own property or an inherited property
用In，当前对象存在或者有继承，就返回true。
hasOwnProperty() --To test whether that object has an own property with the given name. It returns false for inherited properties
用hasOwnProperty() ，只关心本对象，不关心继承来的属性。
propertyIsEnumerable()--The propertyIsEnumerable() refines the hasOwnProperty() test. It returns true only if the named property is an own property and its enumerable attribute is true.
用propertyIsEnumerable() ，和hasOwnProperty() 这个类似，只是要求 属性可枚举。
Instead of using the in operator it is often sufficient to simply query the property and use !== to make sure it is not undefined
o.x !== undefined; // true: o has a property x
替代In的最简单办法就是 query + ！==Undefined
in can distinguish between properties that do not exist and properties that exist but have been set to undefined.
in 有个好处就是还能区分到底属性的值是undefined还是本身就不存在。
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