In JavaScript, Why doesn't for in return the contents of Object.prototype?
I get that using a for ... in loop will return all properties of an object, including those from its prototype chain, and using .keys() will give me an array of non-inher (basically for ... in combined with hasOwnProperty) but why doesn't a for ... in loop include things brought in from Object? ie why doesn't for .. in include the toString function from Object? Thanks!
A property of an object is defined by its key, value and also whether it is configurable, enumerable, writable. A non enumerable property will not show up in for in loops.
When an object property is created like this
var myObject = {};
myObject["a"] = 3;
It is by default configurable, enumerable and writable. It will show up in for in loops. It is possible to create non enumerable properties with Object.defineProperty . You can check if a property is enumerable with .propertyIsEnumerable :
myObject.propertyIsEnumerable("a"); // --> true
Object.propertyIsEnumerable("toString"); // --> false
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