Node calls a zlib callback with (err, undefined) when the operation
fails. The callback adapter passed a NULL Buffer in the result's static
Buffer slot instead: member reads crashed and typeof printed "object".
This applied to deflate, inflate, deflateRaw, inflateRaw, gzip, gunzip
and unzip, with and without options, for callback literals and for
function values.
A function shaped (err, out), with err typed Error | null and out a
Buffer that the body reads, now tests out on entry (bytes.isAbsent).
With a Buffer, the body runs as before. With the NULL Buffer, a second
copy of the body runs with out read as any holding undefined, so typeof,
comparisons, templates, optional chains and console.log give Node's
answers and member reads throw Node's TypeError. Passed on into a Buffer
slot, out stays the NULL Buffer, which a callee with the same entry test
reads as undefined. The parameter keeps its static type, so callers and
adapters are unchanged. A copy that has no such lowering throws SC2020
when it runs.
Conditions on the error parameter fold in the error-path copy, and in
the success copy for a literal that only the adapter calls. Inside an
error-path copy, a nested callback lowers once with its result read as
any, so nested callbacks grow linearly in code size.
Chris Tate
9a73f63b06