ahbeng/NUSMods

import type { VercelRequest, VercelResponse } from '@vercel/node';

export type Request = VercelRequest;
export type Response = VercelResponse;
export type Handler = (req: VercelRequest, res: VercelResponse) => void | Promise<void>;

export type MethodHandlers = {
  GET?: Handler;
  HEAD?: Handler;
  POST?: Handler;
  PUT?: Handler;
  DELETE?: Handler;
  CONNECT?: Handler;
  OPTIONS?: Handler;
  TRACE?: Handler;
  PATCH?: Handler;
};

const handleOptions: Handler = (_req, res) => {
  res.send(200);
};

export const createRouteHandler =
  (
    methodHandlers: MethodHandlers,
    fallback: (mh: MethodHandlers) => Handler,
    rescue: (err: Error) => Handler,
  ): Handler =>
  async (req: Request, res: Response): Promise<void> => {
    try {
      const handler = getHandlerByMethod(
        {
          // HACK: Just insert an OPTIONS handler here to enable CORS for all our
          // serverless functions.
          OPTIONS: handleOptions,
          ...methodHandlers,
        },
        req.method,
      );
      if (handler === undefined) {
        await fallback(methodHandlers)(req, res);
        return;
      }
      await handler(req, res);
    } catch (err) {
      rescue(err)(req, res);
    }
  };

export const defaultFallback =
  (methodHandlers: MethodHandlers) =>
  async (_req: Request, res: Response): Promise<void> => {
    const allowedMethods = Object.entries(methodHandlers).reduce((acc, [method, handler]) => {
      if (handler !== undefined) {
        if (acc === '') return method;
        return `${acc}, ${method}`;
      }
      return acc;
    }, '');
    res.setHeader('Allow', allowedMethods);
    res.status(405).json({
      message: 'Method not allowed',
    });
  };

export const defaultRescue =
  (enableLogging: boolean) =>
  (err: Error) =>
  async (_req: Request, res: Response): Promise<void> => {
    if (enableLogging) {
      // eslint-disable-next-line no-console
      console.error(err);
    }
    res.status(500).json({
      message: 'An unexpected error occurred',
    });
  };

const getHandlerByMethod = (
  methodHandlers: MethodHandlers,
  methodName = '',
): Handler | undefined => {
  const entry = Object.entries(methodHandlers).find(([method]) => method === methodName);
  return entry !== undefined ? entry[1] : undefined;
};