javascript 35 lines · 8 steps

Conditional GET caching in Express

An Express route serves an article but returns 304 Not Modified when the client's cached copy is still fresh.

Explained by highlit
Express Intermediate http-caching conditional-get routing error-handling
1const express = require('express');
2const router = express.Router();
3 
4router.get('/articles/:slug', async (req, res, next) => {
5 try {
6 const article = await Article.findOne({ slug: req.params.slug });
7 
8 if (!article) {
9 return res.status(404).json({ error: 'Article not found' });
10 }
11 
12 const lastModified = article.updatedAt;
13 res.set('Last-Modified', lastModified.toUTCString());
14 res.set('Cache-Control', 'public, max-age=0, must-revalidate');
15 
16 const ifModifiedSince = req.get('If-Modified-Since');
17 if (ifModifiedSince) {
18 const since = new Date(ifModifiedSince);
19 if (!Number.isNaN(since.getTime()) && lastModified.getTime() <= since.getTime() + 999) {
20 return res.status(304).end();
21 }
22 }
23 
24 res.json({
25 slug: article.slug,
26 title: article.title,
27 body: article.body,
28 updatedAt: lastModified.toISOString(),
29 });
30 } catch (err) {
31 next(err);
32 }
33});
34 
35module.exports = router;
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Sending Last-Modified lets clients revalidate cheaply instead of refetching unchanged data.
  2. 2A 304 response has no body, so you save bandwidth when nothing has changed.
  3. 3Comparing timestamps needs a small tolerance because HTTP dates have second-level precision.

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