nim js: appending a uninitialized string appends a null char '\0'

[timotheecour]

I was puzzled by the fact that my js stacktraces were missing depending on some obscure conditions.
It turns out it's caused by 2 bugs:

bug 1: nim js: appending a uninitialized string appends a null char '\0'

this is related to #11783 but seems different, as the bug in #11783 was only present when both appended and appendee were uninitialized, which isn't the case here.

Example

proc fun(): string =
  # result = "" # this removes the bug
  discard

var ret = "foo1"
ret.add fun()
# ret.add string.default # ditto with this
# ret.add "" # ditto with this
ret.add "foo2"
echo (ret, )
# using charCodeAt: reveals a 0
log(ret,)
doAssert false, ret

Current Output

nim js -r main.nim

("foo1\x00foo2",)
foo1foo2
/Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.js:476
    throw new Error(cbuf_294801);
    ^

Error: Error: unhandled exception: /Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.nim(17, 10) `false` foo1
# backtrace doesn't show!

Expected Output

("foo1\x00foo2",) should be ("foo1foo2",), and backtrace shouldn't truncate:

("foo1foo2",)
foo1foo2
/Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.js:465
    throw new Error(cbuf_294801);
    ^

Error: Error: unhandled exception: /Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.nim(17, 10) `false` foo1foo2 [AssertionError]
Traceback (most recent call last)
assertions.nim:27 assertions.failedAssertImpl
assertions.nim:20 assertions.raiseAssert
fatal.nim:39 sysFatal.sysFatal

    at unhandledException (/Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.js:465:11)
    at raiseException (/Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.js:266:3)
    at sys_fatal_288262 (/Users/timothee/git_clone/nim/timn/tests/nim/all/t0552.js:674:3)
...

bug 2:

the fact that the backtrace is truncated (missing all the stackframes) is actually a node bug, which i just filed here: nodejs/node#28761 ; it's just been confirmed
EDIT: that node bug itself just got fixed

Additional Information

• Your Nim version (output of nim -v).
  latest devel f50e450

[timotheecour]

fixed by @hlaaftana 's awesome PR #14158; there's a new regression though (timotheecour#135), but it's unrelated:

Traceback (most recent call last)
assertions.nim:27 assertions.failedAssertImpl
assertions.nim:20 assertions.raiseAssert
fatal.nim:39 sysFatal.sysFatal

prints as:

Traceback (most recent call last)
module tscratch, line: 22
assertions.failedAssertImpl, line: 29
assertions.raiseAssert, line: 22
sysFatal.sysFatal, line: 49