![]() ![]() This problem can be fixed, by directly referring to hasOwnProperty(). TypeError: Property 'hasOwnProperty' of object # is not a function However, if one adds a property to obj whose name is "hasOwnProperty" then that property overrides the method () and getOwnProperty() ceases to work: no guarantees about synchronization or atomicity properties of this method. The function getOwnProperty() invoked the method hasOwnProperty() on obj. The behavior of a map is not specified if the value of an object is changed. Pitfall 2: overriding and invoking methods With that function, the inherited property toString is ignored: There is no built-in way in JavaScript to only read own properties, but you can easily implement one yourself: ![]() The normal way of getting properties accesses all properties: Properties foo added by assigning to obj or obj.foo are enumerable by default. > Object.getOwnPropertyNames(Object.prototype) You can observe the difference by applying the methods to Object.prototype: If you want to list all properties, you need to use Object.getOwnPropertyNames(). This method only returns enumerable properties. The reason that no properties of Object.prototype show up here is that all of them are non-enumerable: > for (propName in obj) console.log(propName)Īlas, it considers inherited enumerable properties. How do we find out all of the keys in obj interpreted as a map? for-in looks promising: hasOwnProperty() provides the necessary services:Ĭollecting property names. Given that obj should be considered empty, we need the check to ignore inherited properties. True // not ok, inherited from Object.prototype The in operator checks whether an object has a property with a given name, but it considers inherited properties: ![]() Let’s see what operations fail to do so.Ĭhecking whether a property exists. obj should be interpreted as an empty map. Obj is an object with no own properties whose prototype is objProto, an object with the property superProp. As a running example, consider the object stored in the variable obj: On the other hand, methods that access only the own (direct) properties of an object and thus ignore inherited properties. On one hand, methods that access the whole prototype chain of an object and thus “see” inherited properties. Question: When dragging a data service from the Panel to the storyboard, I get the error IE Script. When it comes to reading properties, there are two kinds of methods in JavaScript. IE Script Error : Object Doesn't Support this property or method. Pitfall 1: inheritance and reading properties This post points out three pitfalls when doing so. One commonly uses objects as maps from strings to values. JavaScript is Spartan when it comes to built-in data structures. ![]()
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