vercel.json

{
  "public": true,
  "builds": [
    {
      "src": "vercel_basic_auth.js",
      "use": "@vercel/node"
    }
  ],
  "routes": [
    {
      "src": "/.*",
      "dest": "vercel_basic_auth.js"
    }
  ]
}

vercel_basic_auth.js

const protect = require('static-auth');
const safeCompare = require('safe-compare');

const app = protect(
  '/',
  (username, password) => safeCompare(username, process.env.username) && safeCompare(password, process.env.password),
  {
    directory: __dirname + '/public',
    realm: 'vercel-basic-auth.node-static-auth',
    onAuthFailed: res => {
      res.end('Authentication failed.');
    }
  }
);

module.exports = app;