koa-connect/index.ts

Use Connect/Express middleware in Koa

import { Context, Middleware } from 'koa';
import { IncomingMessage, ServerResponse } from 'http';

type ConnectMiddleware = (
  req: IncomingMessage,
  res: ServerResponse,
  callback: (...args: unknown[]) => void
) => void;

const noop = () => {};

/**
 * If the middleware function does not declare receiving the `next` callback
 * assume that it's synchronous and invoke `next` ourselves.
 */
function noCallbackHandler(
  ctx: Context,
  connectMiddleware: ConnectMiddleware,
  next: (err?: unknown) => Promise<void>
): Promise<void> {
  connectMiddleware(ctx.req, ctx.res, noop);
  return next();
}

/**
 * The middleware function does include the `next` callback so only resolve
 * the Promise when it's called. If it's never called, the middleware stack
 * completion will stall.
 */
function withCallbackHandler(
  ctx: Context,
  connectMiddleware: ConnectMiddleware,
  next: (err?: unknown) => Promise<void>
): Promise<void> {
  return new Promise<void>((resolve, reject) => {
    connectMiddleware(ctx.req, ctx.res, (err?: unknown) => {
      if (err) reject(err);
      else resolve(next());
    });
  });
}

/**
 * Returns a Koa middleware function that varies its async logic based on if the
 * given middleware function declares at least 3 parameters, i.e. includes
 * the `next` callback function.
 */
function koaConnect(connectMiddleware: ConnectMiddleware): Middleware {
  const handler =
    connectMiddleware.length < 3 ? noCallbackHandler : withCallbackHandler;
  return function koaConnect(ctx: Context, next: () => Promise<void>) {
    return handler(ctx, connectMiddleware, next);
  };
}

export = koaConnect;