A follow-up to the post https://funtoimagine.com/blog/using-react-trpc-electron/ detailing the resolveIPCResponse function that main.ts uses

trpc-internals.ts

/** Including the MIT license notice verbatim from the trpc repository... **/
/*
MIT License

Copyright (c) <year> <author>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/
  
import {AnyRouter, inferRouterContext, inferRouterError, ProcedureType, Subscription, TRPCError} from "@trpc/server";
import {TRPCErrorResponse, TRPCResponse, TRPCResultResponse} from "@trpc/server/rpc";
import {OnErrorFunction} from "@trpc/server/dist/declarations/src/internals/OnErrorFunction";

export async function callProcedure<
    TRouter extends AnyRouter<TContext>,
    TContext,
    >(opts: {
    path: string;
    input: unknown;
    router: TRouter;
    ctx: TContext;
    type: ProcedureType;
}): Promise<unknown | Subscription<TRouter>> {
    const { type, path, input } = opts;

    const caller = opts.router.createCaller(opts.ctx);
    if (type === 'query') {
        return caller.query(path, input as any);
    }
    if (type === 'mutation') {
        return caller.mutation(path, input as any);
    }
    if (type === 'subscription') {
        const sub = (await caller.subscription(path, input as any)) as Subscription;
        return sub;
    }
    /* istanbul ignore next */
    throw new Error(`Unknown procedure type ${type}`);
}

function transformTRPCResponseItem(
    router: AnyRouter,
    item: TRPCResponse,
): TRPCResponse {
    if ('error' in item) {
        return {
            ...item,
            error: router._def.transformer.output.serialize(item.error),
        };
    }
    if (item.result.type !== 'data') {
        return item;
    }
    return {
        ...item,
        result: {
            ...item.result,
            data: router._def.transformer.output.serialize(item.result.data),
        },
    };
}

/**
 * Takes a unserialized `TRPCResponse` and serializes it with the router's transformers
 **/
export function transformTRPCResponse<
    TResponse extends TRPCResponse | TRPCResponse[],
    >(router: AnyRouter, itemOrItems: TResponse) {
    return Array.isArray(itemOrItems)
        ? itemOrItems.map((item) => transformTRPCResponseItem(router, item))
        : transformTRPCResponseItem(router, itemOrItems);
}

export function getMessageFromUnkownError(
    err: unknown,
    fallback: string,
): string {
    if (typeof err === 'string') {
        return err;
    }

    if (err instanceof Error && typeof err.message === 'string') {
        return err.message;
    }
    return fallback;
}

export function getErrorFromUnknown(cause: unknown): TRPCError {
    // this should ideally be an `instanceof TRPCError` but for some reason that isn't working
    // ref https://github.com/trpc/trpc/issues/331
    if (cause instanceof Error && cause.name === 'TRPCError') {
        return cause as TRPCError;
    }
    const err = new TRPCError({
        code: 'INTERNAL_SERVER_ERROR',
        cause,
    });

    // take stack trace from cause
    if (cause instanceof Error) {
        err.stack = cause.stack;
    }
    return err;
}

export async function resolveIPCResponse<TRouter extends AnyRouter>(opts: {
    createContext: () => Promise<inferRouterContext<TRouter>>;
    onError?: OnErrorFunction<TRouter, IpcRpcRequest>;
    batching?: {
        enabled: boolean;
    };
    router: TRouter;
    req: IpcRpcRequest;
    error?: TRPCError | null;
}): Promise<TRPCResponse | TRPCResponse[]> {
    const {createContext, onError, router, req} = opts;
    const batchingEnabled = opts.batching?.enabled ?? true;
    let ctx: inferRouterContext<TRouter> | undefined = undefined;
    let paths: string[] | undefined = undefined;

    const isBatchCall = !!req.isBatch;
    type TRouterError = inferRouterError<TRouter>;
    type TRouterResponse = TRPCResponse<unknown, TRouterError>;

    function endResponse(
        untransformedJSON: TRouterResponse | TRouterResponse[],
        errors: TRPCError[],
    ): TRPCResponse | TRPCResponse[] {
        return transformTRPCResponse(router, untransformedJSON);
    }

    const {type, path, input, id, context} = req;

    try {
        if (opts.error) {
            throw opts.error;
        }
        if (isBatchCall && !batchingEnabled) {
            throw new Error(`Batching is not enabled on the server`);
        }
        // @ts-expect-error no unknown string in union
        if (type === 'unknown' || type === 'subscription') {
            throw new TRPCError({
                message: `Unexpected request method ${type}`,
                code: 'METHOD_NOT_SUPPORTED',
            });
        }
        const rawInput = input;

        paths = isBatchCall ? path.split(',') : [path];
        ctx = await createContext();

        const deserializeInputValue = (rawValue: unknown) => {
            return typeof rawValue !== 'undefined'
                ? router._def.transformer.input.deserialize(rawValue)
                : rawValue;
        };
        const getInputs = (): Record<number, unknown> => {
            if (!isBatchCall) {
                return {
                    0: deserializeInputValue(rawInput),
                };
            }

            if (
                rawInput == null ||
                typeof rawInput !== 'object' ||
                Array.isArray(rawInput)
            ) {
                throw new TRPCError({
                    code: 'BAD_REQUEST',
                    message: '"input" needs to be an object when doing a batch call',
                });
            }
            const input: Record<number, unknown> = {};
            for (const key in rawInput) {
                const k = key as any as number;
                const rawValue = (rawInput as any)[k];

                const value = deserializeInputValue(rawValue);

                input[k] = value;
            }
            return input;
        };
        const inputs = getInputs();

        const rawResults = await Promise.all(
            paths.map(async (path, index) => {
                const input = inputs[index];
                try {
                    const output = await callProcedure({
                        ctx,
                        router: router,
                        path,
                        input,
                        type,
                    });
                    return {
                        input,
                        path,
                        data: output,
                    };
                } catch (cause) {
                    const error = getErrorFromUnknown(cause);

                    onError?.({error, path, input, ctx, type: type, req});
                    return {
                        input,
                        path,
                        error,
                    };
                }
            }),
        );
        const errors = rawResults.flatMap((obj) => (obj.error ? [obj.error] : []));
        const resultEnvelopes = rawResults.map((obj) => {
            const {path, input} = obj;

            if (obj.error) {
                const json: TRPCErrorResponse<TRouterError> = {
                    id: null,
                    error: router.getErrorShape({
                        error: obj.error,
                        type,
                        path,
                        input,
                        ctx,
                    }),
                };
                return json;
            } else {
                const json: TRPCResultResponse<unknown> = {
                    id: null,
                    result: {
                        type: 'data',
                        data: obj.data,
                    },
                };
                return json;
            }
        });

        const result = isBatchCall ? resultEnvelopes : resultEnvelopes[0];
        return endResponse(result, errors);
    } catch (cause) {
        // we get here if
        // - batching is called when it's not enabled
        // - `createContext()` throws
        // - post body is too large
        // - input deserialization fails
        const error = getErrorFromUnknown(cause);

        const json: TRPCErrorResponse<TRouterError> = {
            id: null,
            error: router.getErrorShape({
                error,
                type,
                path: undefined,
                input: undefined,
                ctx,
            }),
        };
        onError?.({
            error,
            path: undefined,
            input: undefined,
            ctx,
            type: type,
            req,
        });
        return endResponse(json, [error]);
    }
}

export interface IpcRpcRequest {
    id: number;
    type: "query" | "mutation" | "subscription";
    path: string;
    input: unknown;
    context: unknown;
    isBatch?: boolean;
}