byte[] does not work as type parameter when return value is Uint8array

[rbsdc]

Describe the bug When byte[] is used as type parameter and an Uint8Array is used as return value in JavaScript, then an error is thrown. If byte[] is used directly, it works fine with Uint8Array. Mentioned in a comment here: https://github.com/google/j2cl/issues/144.

To Reproduce The following code does work when transpiled to JavaScript:

interface FileReader {
  byte[] readRange(int offset, int length);
}

class FileTest {
  public void doSomething(FileReader reader) {
    byte[] bytes = reader.readRange(0, 10);
  }
}

In Javascript:

class FileReader {
   readRange(offset, length) {
      return new Uint8Array([1, 2]); 
  }
 let fileTest = new FileTest();
 fileTest.doSomething(new FileReader()) // works
}

The following does not work:

@JsType
public interface Thenable {
    @JsFunction
    interface FullFilledCallback {
        void execute(byte[] o);
    }

    @JsFunction
    interface RejectedCallback<T> {
        void execute(byte[] o);
    }

    void then(FullFilledCallback onFulfilled, RejectedCallback onRejected);
}

interface FileReader {
  Thenable<byte[]> readRange(int offset, int length);
}

class FileTest {
  public void doSomething(FileReader reader) {
    reader.readRange(0, 10).then(bytes -> 
       //do something with the bytes
    );
  }
}

In Javascript:

class FileReader {
   readRange(offset, length) {
      return new Promise((resolve, reject) => {
         resolve(new Uint8Array([1, 2]));
      }); 
  }
 let fileTest = new FileTest();
 fileTest.doSomething(new FileReader()) // throws something similar to Error: Class$obf_1003: Class$obf_1001 cannot be cast to [LClass$obf_1002]...
}

Bazel version 5.1.0

[gkdn]

This is because byte[] is not a Uint8Array but it is a Array<number> with extra metadata. We are considering mapping primitive arrays to TypedArray but that's not going to be soon.

If you really would like to use byte[] in JS, one option you have is to create a static function that returns a byte[]. Either the function can take byte parameters and return constructed array: static byte[] createByteArray(byte... bytes){ return bytes;} or you can manipulate returned empty array to add items.

If you care about using Uint8Array from Java side, then you can use Elemental2 or JsArrayLike from the jsinterop.base library.

[rbsdc]

Thank you for the clarification. I already thought that Uint8Array might not be the correct byte[] equivalent, but wondered anyhow why it did work when using a plain byte[] parameter (as described above and in the other issue). Anyway, by using your suggested method static byte[] createByteArray(byte... bytes){ return bytes;} and manipulating the returned array I made it work. Thank you again!