ECMAScript Proposal, specs, and reference implementation for Error.isError
Spec drafted by @ljharb.
This proposal is currently Stage 2 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.
This list is not exhaustive.
throw
statement can throw any exception with an unknown type of data, and a verification is required for the data to be collected. The thrownData instanceof Error
doesn't satisfy Error
because the thrown error will inherit the Error
instance of another realm, meaning contentWindow.Error
is the abstract and thrownData instanceof document.getElementsByTagName('iframe')[0].contentWindow.Error
is required to satisfy the validation.You can view the spec rendered as HTML.