Stream: jco

Topic: Jco expects one extra parameter in imported async function

Elmer Bulthuis (Jul 05 2025 at 19:37):

Hi there! I am working with wit-bindgen, wasm-tools and jco. Now everything works fine, until I introduce this interface to my wit file:

interface imports {
  enum fetch-text-error {
    io-error,
    http-error,
  }

  fetch-text: async func(location: string) -> result<string, fetch-text-error>;
}

I generate a wasm component like this:

wasm-tools component new --output target/wasm32-unknown-unknown/release/jns42_core.component.wasm target/wasm32-unknown-unknown/release/jns42_core.wasm

Then I generate the js/ts code like this:

> jco transpile --instantiation async --out-dir dist $npm_config_local_prefix/target/wasm32-unknown-unknown/release/jns42_core.component.wasm

Wich results in:

(jco transpile) ComponentError: failed to extract interface information from component

Caused by:
    type mismatch for export `0` of module instantiation argument ``
    expected: (func (param i32 i32 i32) (result i32))
    found:    (func (param i32 i32) (result i32)) (at offset 0x31aaaa)

Now I assume that the async function in the interface is causing the problems, because if i remove the async keyword everything works fine. So I guess we have a bug!

Now I assume the bug is either in wasm-tools or in jco. I would love to find it, and then set it free into nature so we are not bothered by it anymore. But in order to do that I first need to find it.

I am not too familiar with wasm-tools and or jco (not yet!) and not sure where to look first. So, I have a few questions that might help me get up too speed!

• Is this s a bug?
• Is is more likely that the bug is in jco or in wasm-tools? Wich one of the projects is more mature?
• jco expects 3 parameters for the function! Why is that? I think that 2 parameters are because i send a string. Is the third maybe a result of the thing being async?
• Any other tips for debugging this?

Thanks in advance for the tips! I will be digging around in the wasm-tools / jco project in the meanwhile!

Have a nice evening!

Last updated: Dec 06 2025 at 07:03 UTC