Awesome
Error.isError
ECMAScript Proposal, specs, and reference implementation for Error.isError
Spec drafted by @ljharb.
This proposal is currently Stage 2 of the process.
Rationale
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.
Use cases
This list is not exhaustive.
- debugging: it is very helpful to humans, even if not always to programs, to know what kind of thing a value is. Knowing if something is a "real" native error is thus valuable information to make available, including to error-reporting libraries.
- serialization: platforms such as RunKit need to serialize values safely and reconstruct them or describe them in the user’s browser. brand checking is critical for this.
- structuredClone: this HTML method, which is also in node, brand-checks, and has special behavior for native Errors. JS programs need a way to know in advance if this behavior will be applied
- centralized error collection: reporting errors from an iframe (w/ same origin) to a top frame where all errors are collected, evaluated, and reported to a personalized server to be investigated at a later time. However, a
throw
statement can throw any exception with an unknown type of data, and a verification is required for the data to be collected. ThethrownData instanceof Error
doesn't satisfyError
because the thrown error will inherit theError
instance of another realm, meaningcontentWindow.Error
is the abstract andthrownData instanceof document.getElementsByTagName('iframe')[0].contentWindow.Error
is required to satisfy the validation.
Spec
You can view the spec rendered as HTML.