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.

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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;

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STEP 01

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Three takeaways

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

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