regression(0.18): nim js gives runtime error when appending to an uninitialized string

[timotheecour]

nim js gives runtime error when appending an uninitialized string to an uninitialized string.
This works fine with nim c but gives runtime error with nim js:

reduced from a crash in a more complex program:

Example

proc fun(): string =
  # return "" # this removes the crash
  discard

var ret: string
# ret = ""  # this removes the crash
ret.add fun()

Current Output

/Users/timothee/git_clone/nim/timn/tests/nim/all/t0549.js:39
if (ret_311230[0] != null) { ret_311230[0] = (ret_311230[0]).concat(fun_311214()); } else { ret_311230[0] = fun_311214().slice(); };
                                                                                                                        ^

TypeError: Cannot read property 'slice' of null
    at Object.<anonymous> (/Users/timothee/git_clone/nim/timn/tests/nim/all/t0549.js:39:121)
    at Module._compile (internal/modules/cjs/loader.js:774:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:785:10)
    at Module.load (internal/modules/cjs/loader.js:641:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:837:10)
    at internal/main/run_main_module.js:17:11

Expected Output

should work (and print nothing), just like it does with nim c

Additional Information

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

same bug with 0.19, 0.20

doesn't crash with 0.18, hence marking it as a regression, but not super sure 0.18 was 100% correct either, eg, i can make 0.18 crash by adding echo ret (but doesn't crash if i precede by ret.add "asdf")