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? ``` > err = new Error("an error") [Error: an error] > err.message 'an error' > err.name 'Error' > Object.keys(err) [] > JSON.stringify(err) '{}' ```

Accepted Answer

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.)

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

Solution

Problem