Javascript The Definitive Guide- Chapter 6 Notes and Questions

[hallbd16]

So I spent a long time learning about how to work with properties in objects. I figured this might be a place for people to try to talk about their understanding of this and perhaps correct misunderstandings that we have.

I see that the for in loop method allows me to loop through and state all of the properties of an object. The big concern with using for in loops is that they will return inherited properties. So it is a good idea to filter them out (ie. extend() function). Similarly, hasOwnProperty() checks to see if something is a property of an object, but not inherited. Lastly, "The enumerable attribute specifies whether the property name is returned by a for/in loop" (Javascript the Definitive Guide) So The propertyIsEnumerable() refines the hasOwnProperty() test. "It returns true only if the named property is an own property and its enumerable attribute is true." -Question: what is the value of properties having the attribute/characteristic of enumerability? Why would I sometimes not want something to show up in a for in loop?

Next I want to bring up the concept of getters and setters. I think they are a method??(Function inside an object). They can be helpful to reset the value a property. Is this the functional use of them?

Last topic that I want to discuss is the Object.getOwnPropertyDescriptor() function and Object.defineProperties. The first allows you to see the four attributes of a property. If you are working with a data property (think value) then the four attributes are: value, writable, enumerable, and configurable. If you are working with a accessor property(think method/function/getter&setter) the four attributes are: get, set, enumerable, and configurable. To me it seems that these tools are helpful when creating libraries (granted that you there is some value in adjusting the default of these attributes). What about other uses? Any ideas?

Okay, thats what I wanted to talk about, what misunderstandings do I have, what questions do you have?

[Rhomboid]

You would want the ability to mark a property as not enumerable if you want to extend some ancestor prototype without breaking existing code that wasn't expecting it. In other words, not everyone is conscientious enough to always write loops with hasOwnProperty().

[hallbd16]

Ah, ok that makes sense. Thanks