Basic implementation of https://expressjs.com/en/4x/api.html#express.static, does not support any option.

basic-express-static-middleware.js

const express = require("express"); // https://expressjs.com/en/4x/api.html
const fs = require("fs"); // https://nodejs.org/api/fs.html
const path = require("path"); // https://nodejs.org/api/path.html

/*
 * This function exists for convenience only. It allows to create a new
 * middleware with the given parameters (here we only expect a single one: the
 * root directory). It mimics the genuine express.static function signature
 * (modulo the options argument).
 */
function staticMiddlewareFactory(root) {
  // A middleware is a function with an arity of 3 (see: https://expressjs.com/en/guide/writing-middleware.html)
  return function staticMiddleware(req, res, next) {
    // Strip the leading slash(es) so that it is considered as a relative path.
    // Note that when the middleware is mounted at a specific path, req.url is
    // relative to req.baseUrl. So it always behaves as expected.
    const relativePath = req.url.replace(/^\/*/, "");
    // Compute the absolute path relative to the current working directory
    // (process.cwd()), the root directory, and the path extracted from the url.
    const absolutePath = path.resolve(process.cwd(), root, relativePath);
    // Try to read the file and send it to the client.
    fs.readFile(absolutePath, (error, data) => {
      // If the file is a directory (EISDIR error code) or is not found (ENOENT
      // error code), just continue to the next route without triggering an
      // error. Otherwise pass the error to the next() function to abort the
      // request.
      if (error) {
        return ["EISDIR", "ENOENT"].includes(error.code) ? next() : next(error);
      }
      res.set("content-type", "text/plain"); // TODO: set a proper content-type infered from the file extension
      return res.send(data);
    });
  };
}

express() // Initialize the express server, here we leverage the chained notation
  .use(staticMiddlewareFactory(__dirname)) // Load the middleware, with the root being the directory of this file (see: https://nodejs.org/docs/latest/api/modules.html#modules_dirname)
  .get("/hello", (req, res) => res.send("hello world")) // Make sure the middleware forwards the requests it cannot fulfill to the next middleware/handler
  .listen(3000, "0.0.0.0"); // Start the server