The ESNext error lib is now part of the type world, and Error.isError(value)
compiles. A catch binding tests its payload against the Error class, an
unknown value tests the checked-dynamic error encoding, and a union tests its
runtime tag against the arms whose class extends Error. Other values answer
statically: true for instances of Error and its subclasses, false for
everything else, including objects that only have an error's shape. The
argument is always evaluated. Unions with unknown arms are refused.
Chris Tate
983058a3a7