he/him 
Surprised to find that the existence of Error.prototype.stack isn't in the ecma262 spec. Format makes sense but non existence surprises me.
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@sebmck but yes. it's non-standard and IIRC has a few differences across browsers which makes it hard to write reliable error handling code
@tbreisacher super weird. i expected the stack property to be specified like function toString where up to implementation, at least defined.