In this quick example, we’ll create a middlware for express. The middlware checks every request to ensure it has a valid API key. 
npm i theauthapi express --save

import TheAuthAPI from "theauthapi";
import express from "express";
const app = express();

const theAuthAPI = new TheAuthAPI.default("${accessKey}");

app.use(function (req, res, next) {
  if (req.headers["x-api-key"]) {
    theAuthAPI.apiKeys
      .authenticateKey(req.headers["x-api-key"])
      .then((key) => {
        if (!key) {
          res.status(401).send({ message: "Invalid API key" });
        }
        req.key = key;
        next();
      })
      .catch((err) => {
        res.status(500).send({ message: err.message });
      });
  } else {
    res.status(401).send({
      message: "No API key, be sure to set it as the 'x-api-key' header",
    });
  }
});

app.get("/", (req, res) => {
  res.send(req.key);
});

app.listen(3000);