Uncaught RuntimeError: illegal cast - when trying to access string passed from Javascript to dart wasm

[akalankavinda]

I have exported the following function in my dart wasm code

@pragma("wasm:export")
void foo(JSString x) {
   print(x.toString());
}

but whe I try to invoke this method from the javascript side,
I get the following error:

It points out to the following place in wasm library

Am I doing this the wrong way or is this an issue with compiler ?

PS: I was playing around with the following repo
https://github.com/mit-mit/sandbox/tree/main/demos/webwasm

[srujzs]

Prefer using Function.toJS to export a Dart function.

Here's an example:

import 'dart:js_interop';

@JS()
external set foo(JSFunction f);

void main() {
  void f(JSString x) {
   print(x.toString());
  }
  foo = f.toJS;
}

Here, we wrap a Dart function with a JS function and set it in the property foo in globalThis. So, you can call globalThis.foo('hello world') in JS after this function is set.

[lrhn]

(Maybe also use x.toDart instead of x.toString() to convert a JSString to a Dart String.)

[srujzs]

I missed that. :) Yes, convert it back into a String using toDart. toString will likely do the right thing anyways, but prefer the conversion.

[akalankavinda]

Thanks @srujzs @lrhn for the support.

With your suggestions, I modified my code as follows

import 'dart:js_interop';

@JS()
external set _onKeyup(JSFunction fn);

void main() {
  _onKeyup = _onKeyupDart.toJS;
}

void _onKeyupDart(JSString val) {
  print(val.toDart);
}

Now from the JavaScript side _onKeyup() is globally available and it is working as expected ✅✅