r19/src/client/createRPCClient.ts

Simple remote procedure calls in TypeScript

import { encode, decode } from "@msgpack/msgpack";

import { isErrorLike } from "../lib/isErrorLike";
import { isR19ErrorLike } from "../lib/isR19ErrorLike";
import { replaceLeaves } from "../lib/replaceLeaves";

import { Procedures, Procedure, ProcedureCallServerResponse } from "../types";
import { R19Error } from "../R19Error";

const createArbitrarilyNestedFunction = <T>(
	handler: (path: string[], args: unknown[]) => unknown,
	path: string[] = [],
): T => {
	return new Proxy(() => void 0, {
		apply(_target, _this, args) {
			return handler(path, args);
		},
		get(_target, property) {
			return createArbitrarilyNestedFunction(handler, [
				...path,
				property.toString(),
			]);
		},
	}) as T;
};

// `RPCClient` is currently a clone of `TransformProcedures`, but that could
// change in the future.
export type RPCClient<TProcedures extends Procedures> =
	TransformProcedures<TProcedures>;

type TransformProcedures<TProcedures> =
	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	TProcedures extends Procedures
		? { [P in keyof TProcedures]: TransformProcedures<TProcedures[P]> }
		: // eslint-disable-next-line @typescript-eslint/no-explicit-any
			TProcedures extends Procedure<any>
			? TransformProcedure<TProcedures>
			: TProcedures;

// eslint-disable-next-line @typescript-eslint/no-explicit-any
type TransformProcedure<TProcedure extends Procedure<any>> = (
	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	...args: TransformProcedureArgs<Parameters<TProcedure>> extends any[]
		? TransformProcedureArgs<Parameters<TProcedure>>
		: []
) => Promise<TransformProcedureReturnType<Awaited<ReturnType<TProcedure>>>>;

// eslint-disable-next-line @typescript-eslint/no-explicit-any
type TransformProcedureArgs<TArgs extends any[]> = {
	[P in keyof TArgs]: TransformProcedureArg<TArgs[P]>;
};

type TransformProcedureArg<TArg> = TArg extends
	| Record<string, unknown>
	| unknown[]
	? {
			[P in keyof TArg]: TransformProcedureArg<TArg[P]>;
		}
	: TArg extends Buffer
		? Blob
		: TArg;

type TransformProcedureReturnType<TReturnType> = TReturnType extends
	| Record<string, unknown>
	| unknown[]
	? {
			[P in keyof TReturnType]: TransformProcedureReturnType<TReturnType[P]>;
		}
	: TReturnType extends Buffer
		? Blob
		: TReturnType extends Error
			? {
					name: string;
					message: string;
				}
			: TReturnType;

export type ResponseLike = {
	arrayBuffer: () => Promise<ArrayBuffer>;
};
export type FetchLike = (
	input: string,
	init: {
		method: "POST";
		// eslint-disable-next-line @typescript-eslint/no-explicit-any
		body: any;
		headers: Record<string, string>;
	},
) => Promise<ResponseLike>;

export type CreateRPCClientArgs = {
	serverURL: string;
	fetch?: FetchLike;
};

export const createRPCClient = <TProcedures extends Procedures>(
	args: CreateRPCClientArgs,
): RPCClient<TProcedures> => {
	const resolvedFetch: FetchLike =
		args.fetch || globalThis.fetch.bind(globalThis);

	return createArbitrarilyNestedFunction(
		async (procedurePath, procedureArgs) => {
			const preparedProcedureArgs = await replaceLeaves(
				procedureArgs,
				async (value) => {
					if (value instanceof Blob) {
						return new Uint8Array(await value.arrayBuffer());
					}

					if (typeof value === "function") {
						throw new R19Error("r19 does not support function arguments.", {
							procedurePath,
							procedureArgs,
						});
					}

					return value;
				},
			);

			const body = encode(
				{
					procedurePath: procedurePath,
					procedureArgs: preparedProcedureArgs,
				},
				{ ignoreUndefined: true },
			);

			const res = await resolvedFetch(args.serverURL, {
				method: "POST",
				body,
				headers: {
					"Content-Type": "application/msgpack",
				},
			});

			const arrayBuffer = await res.arrayBuffer();
			const resObject = decode(
				new Uint8Array(arrayBuffer),
			) as ProcedureCallServerResponse;

			if ("error" in resObject) {
				const resError = resObject.error;

				if (isR19ErrorLike(resError)) {
					const error = new R19Error(resError.message, {
						procedurePath,
						procedureArgs,
					});
					error.stack = resError.stack;

					throw error;
				} else if (isErrorLike(resError)) {
					const error = new Error(resError.message);
					error.name = resError.name;
					error.stack = resError.stack;

					throw error;
				} else {
					throw new R19Error(
						"An unexpected response was received from the RPC server.",
						{
							procedurePath,
							procedureArgs,
							cause: resObject,
						},
					);
				}
			} else {
				return replaceLeaves(resObject.data, async (value) => {
					if (value instanceof Uint8Array) {
						return new Blob([value]);
					}

					return value;
				});
			}
		},
	);
};