JavaScript string-reverse example fails

[catamorphism]

I'm trying to work through the "Creating Components" > "JavaScript" section. Under the Exporting WIT Interfaces with jco section, I'm finding that the string-reverse example isn't working.

Steps to reproduce:

1. Created a wit/component.wit file as instructed, except that I had to edit it to fix a syntax error (missing semicolon after the package declaration).
2. Created a string-reverse.mjs file as instructed.
3. Ran the npx jco componentize command as instructed; note that I had to change --disable all to --disable=all.
4. Ran the npx jco transpile command as instructed.
5. Added a package.json file that looks like:

•  {
   "name": "string-reverse",
   "description": "Simple codebase for compiling a string reversing interface to WebAssembly with jco",
   "type": "module"
   }
  

  (This step isn't in the docs, but if it's omitted, I get an error about modules when running the next step.)

6. Saved the next bit of code (beginning with import { reverse } to run.js.
7. node run.js

Result:

wasm://wasm/02a47f8a:1


RuntimeError: unreachable
    at wasm://wasm/02a47f8a:wasm-function[3]:0x971c
    at wasm://wasm/02a47f8a:wasm-function[4342]:0x1b4560
    at example:string-reverse/reverse@0.1.0#reverse-string (wasm://wasm/02a47f8a:wasm-function[13444]:0x81449e)
    at Object.reverseString (file:///home/tjc/wasm-scratch/js-example/string-reverse-2/dist/transpiled/string-reverse.js:668:15)
    at file:///home/tjc/wasm-scratch/js-example/string-reverse-2/run.js:4:26

Node.js v22.18.0

I'll keep trying to debug this, but I was hoping someone else might be able to try reproducing it so I know whether it's some quirk of my setup.

npx jco --version prints "1.13.2".

[catamorphism]

If I use the example in the jco repo and follow the instructions there, it works. I'll keep digging...

[catamorphism]

I see that the code in this repo is different from the code in the jco repo. This repo: (comments removed so it's easier to follow)

function reverseString(s) {
  return s.reverse();
}

export const reverse = {
    reverseString,
};

jco repo:

export const reverse = {
 reverseString(s) {
    return s.split("")
      .reverse()
      .join("");
  }
};

I don't know why the second one works and the first one crashes. I can change the example in the book to match the jco one, but that's rather unsatisfying.

[itowlson]

Might be worth raising as an issue on jco - they might be able to guide us if it's expected (and should be fixed in the book) - and if so why - or a bug (and should get fixed in jco).

[vados-cosmonic]

Thanks for finding this @catamorphism -- it's a bit weird! I was really confused until I realized... There is no String.reverse, as much as there should be.

Unfortunately the error you see when you try to access a method that doesn't exist (or some other undefined thing) is the one you got -- IMO that does need to be made more legible, but that's a runtime level problem.

[catamorphism]

@vados-cosmonic Got it! I have this fixed in the docs and will be submitting a PR.

[catamorphism]

Fixed in #304