Using HTTP ETag to Control Cache in a Koa Server Example

etag-cache-server-example.mts

import cors from '@koa/cors';
import Router from '@koa/router';
import fs from 'fs';
import https from 'https';
import Koa from 'koa';
import { DateTime } from 'luxon';

const app = new Koa();
const router = new Router();

let etag = (Math.random() * 1000000).toString();
let lastModifiedDate = DateTime.now().toHTTP();

router.get('/etag', async (ctx) => {
  const ifNoneMatch = ctx.request.headers['if-none-match'];

  if (Math.random() > 0.8) {
    etag = (Math.random() * 1000000).toString();
    lastModifiedDate = DateTime.now().toHTTP();
  }

  const notModifed =
    !!ifNoneMatch &&
    ifNoneMatch.substring(
      ifNoneMatch.indexOf('W/') === 0 ? 3 : 1,
      ifNoneMatch.length - 1,
    ) === etag;

  if (notModifed) {
    ctx.response.status = 304;
    ctx.response.set('Last-Modified', lastModifiedDate);
    ctx.response.set('Etag', `W/"${etag}"`);
  } else {
    ctx.response.set('Last-Modified', lastModifiedDate);
    ctx.response.set('Etag', `W/"${etag}"`);
    ctx.body = { v: etag };
  }
});

app.use(cors()).use(router.routes()).use(router.allowedMethods());

https
  .createServer(
    {
      key: fs.readFileSync(process.env.LOCALHOST_HTTPS_KEY!),
      cert: fs.readFileSync(process.env.LOCALHOST_HTTPS_CERT!),
    },
    app.callback(),
  )
  .listen(9899);