--output-match crashes when a group did not take part in the match #684

--output-match (-m) crashes if the pattern has a group that is not part of a match, which is the normal case for alternatives and optional groups.

Seen with rexreplace 7.1.14 on Node v20.20.2.

Reproduce

> echo foobar > my.file
> rexreplace '(f)|(b)' _ my.file -m

Actual

fnode:internal/streams/writable:482
      throw new ERR_INVALID_ARG_TYPE(
      ^

TypeError [ERR_INVALID_ARG_TYPE]: The "chunk" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined

Expected

f
b

Cause

The replacer function in getFinalReplacement() in src/engine.ts (line 242) writes each group to stdout without checking if it is undefined:

for (var i = 1; i < arguments.length - 2; i++) {
	process.stdout.write(arguments[i]);
}

Test

The JS test suite being added on the js-test branch has a test.todo describing the correct behaviour in test/js/output-match.test.js:

BUG: a group that did not take part in the match prints nothing