ECMAScript Proposal, specs, and reference implementation for Error.isError

Spec drafted by @ljharb.

This proposal is currently withdrawn from stage 0 of the process.


I brought up concerns to the committee about Symbol.toStringTag, and how previously reliable and unspoofable Object#toString calls would now no longer be reliable. The committee consensus was that as long as there were prototype methods for all builtins that, at the least, threw an error when an internal slot was not present, that would be sufficient to serve as a reliable branding test.

However, the internal slot for Error instances (and its subclasses) is only checked in Object#toString itself - leaving no possible reliable test in a world with Symbol.toStringTag.

instanceof Error, of course, is unreliable because it will provide a false negative with a cross-realm (eg, from an iframe, or node's vm modules) Error instance.


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