demo · v134 · javascript

Test Suite

Sixteen pathological inputs. For each, we run Error.isError(x), x instanceof Error, Object.prototype.toString.call(x) === "[object Error]", and x.constructor?.name?.endsWith("Error"). Side-by-side scoreboard so you can see which check actually does what people expect.

input	Error.isError	instanceof Error	toString tag	name endsWith

why the other checks fail

instanceof Error — false for errors from other realms (iframe, Worker, vm context). The constructor's prototype chain is a different Error.prototype object.
toString tag — works for native errors but is forgeable via Symbol.toStringTag.
name endsWith "Error" — useful heuristic but trips on anything called MyCustomError that doesn't actually derive from Error, and misses DOMException.
Error.isError — defined by spec to check the internal [[ErrorData]] slot. True only for real errors regardless of realm, false for fakes regardless of how convincing.

why the other checks fail

see also