var e = new Error('bla'); console.log(Object.getOwnPropertyNames(e).length) // 0, should be 1 console.log(e.message) // 'bla' as expected console.log(Object.getOwnPropertyNames(e).length) // 1 as expected I imagine that getting the property makes it appear in some way. In any case, the second line has an unexpected behavior.

In case it's not obvious, in both cases, the Object.getOwnPropertyNames(e) should be ['message']

Also, check if the bug is in other error types just in case ;-)

Hu, interesting. I bet Waldo has some idea of what's going on.