How to throw/catch custom error with comlink. typescript

Question

In UI process, I want to catch custom Error throw in Webworker with comlink.

The custom Error is extended Error, so comlink serialize/deserialize to Error not to custom Error.

serialize

if (value instanceof Error) {
  serialized = {
    isError: true,
    value: {
      message: value.message,
      name: value.name,
      stack: value.stack,
    },
  };
}

https://github.com/GoogleChromeLabs/comlink/blob/520e4d64072bc07c3c60dd67dd95910a23602445/src/comlink.ts#L248

deserialize

if (serialized.isError) {
  throw Object.assign(
    new Error(serialized.value.message),
    serialized.value
  );
}

https://github.com/GoogleChromeLabs/comlink/blob/520e4d64072bc07c3c60dd67dd95910a23602445/src/comlink.ts#L265

I tried to customize throwTransferHandler to throw custom Error, but It did not work. Because throwMarker is unique Symbol and not exported.

canHandle: (value): value is ThrownValue =>
  isObject(value) && throwMarker in value,

https://github.com/GoogleChromeLabs/comlink/blob/520e4d64072bc07c3c60dd67dd95910a23602445/src/comlink.ts#L245

Answer 1

I ended up defining local module which reexports comlink and use serialize-error as custom error serialization/deserialization

import * as Comlink from "comlink";
import ErrorCodec from "serialize-error";
export * from "comlink";

interface ThrownValue {
  value: unknown;
}
type SerializedThrownValue =
  | { isError: true; value: ErrorCodec.ErrorObject }
  | { isError: false; value: unknown };

const throwTransferHandler_:
  | Comlink.TransferHandler<unknown, unknown>
  | undefined = Comlink.transferHandlers.get("throw");

const throwTransferHandler = throwTransferHandler_ as Comlink.TransferHandler<
  ThrownValue,
  SerializedThrownValue
>;

const throwTransferHandlerCustom: Comlink.TransferHandler<
  ThrownValue,
  SerializedThrownValue
> = {
  canHandle: throwTransferHandler.canHandle,
  serialize: ({ value }) => {
    let serialized: SerializedThrownValue;
    if (value instanceof Error) {
      serialized = {
        isError: true,
        value: ErrorCodec.serializeError(value),
      };
    } else {
      serialized = { isError: false, value };
    }
    return [serialized, []];
  },
  deserialize: (serialized) => {
    if (serialized.isError) {
      const error = ErrorCodec.deserializeError(serialized.value);
      throw error;
    }
    throw serialized.value;
  },
};

Comlink.transferHandlers.set("throw", throwTransferHandlerCustom);

You should make sure that this version of module is imported in all other places instead of direct comlink import

---

As an alternative In canHandle you can also use getOwnPropertySymbols and toString:

Object.getOwnPropertySymbols({[Symbol("Comlink.thrown")]: 1})
  .some(_ => _.toString() === 'Symbol(Comlink.thrown)')