Why can't I see the keys of an Error object?

Question

I am mystified by the fact that when I create a new Error object I can see its message or name, but I can\'t see a list of its keys by using the standard ways. Why is that?

Answer 1

JavaScript properties may be non-enumerable, which means they does not appear in for..in loops or Object.keys results.

You can use Object.getOwnPropertyNames to get all properties (enumerable or non-enumerable) directly on an object. I say "directly" because normal enumeration looks up the object's prototype chain to get enumerable properties on parent prototypes, while getOwnPropertyNames does not.

Thus, Object.getOwnPropertyNames(err) only shows

['stack',
 'arguments',
 'type',
 'message']

The name property is a non-enumerable property of Error.prototype and is never set directly on an Error instance. (Prototyping recap: when you try to access err.name, the lookup err turns up nothing, so the interpreter looks at Error.prototype, which does have a name property.)