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中判斷一個對象是否包含某個屬性，可以使用 in，hasOwnProperty() and propertyIsEnumerable()

or simply by querying the property.

或者直接使用查詢屬性。

in--It returns true if the object has an own property or an inherited property 

用In，當前對象存在或者有繼承，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() ，只關心本對象，不關心繼承來的屬性。

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() 這個類似，只是要求 屬性可枚舉。

 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的最簡單辦法就是  query   +    ！==Undefined

in can distinguish between properties that do not exist and properties that exist but have been set to undefined.

in 有個好處就是還能區(qū)分到底屬性的值是undefined還是本身就不存在。

以上這篇JS Testing Properties 判斷屬性是否在對象里的方法就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支持腳本之家。

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