Use if-last-modified in dev server (#8166)

packages/reporters/dev-server/package.json

@@ -37,6 +37,7 @@
     "@parcel/types": "2.6.0",
     "connect": "^3.7.0",
     "ejs": "^3.1.6",
+    "fresh": "^0.5.2",
     "http-proxy-middleware": "^2.0.1",
     "launch-editor": "^2.3.0",
     "mime-types": "2.1.18",

packages/reporters/dev-server/src/Server.js

@@ -33,6 +33,7 @@ import {getHotAssetContents} from './HMRServer';
 import nullthrows from 'nullthrows';
 import mime from 'mime-types';
 import launchEditor from 'launch-editor';
+import fresh from 'fresh';
 
 function setHeaders(res: Response) {
   res.setHeader('Access-Control-Allow-Origin', '*');
@@ -44,6 +45,7 @@ function setHeaders(res: Response) {
     'Access-Control-Allow-Headers',
     'Origin, X-Requested-With, Content-Type, Accept, Content-Type',
   );
+  res.setHeader('Cache-Control', 'max-age=0, must-revalidate');
 }
 
 const SOURCES_ENDPOINT = '/__parcel_source_root';
@@ -343,6 +345,12 @@ export default class Server {
       return;
     }
 
+    if (fresh(req.headers, {'last-modified': stat.mtime.toUTCString()})) {
+      res.statusCode = 304;
+      res.end();
+      return;
+    }
+
     return serveHandler(
       req,
       res,

yarn.lock

@@ -6095,6 +6095,11 @@ fragment-cache@^0.2.1:
   dependencies:
     map-cache "^0.2.2"
 
+fresh@^0.5.2:
+  version "0.5.2"
+  resolved "https://registry.yarnpkg.com/fresh/-/fresh-0.5.2.tgz#3d8cadd90d976569fa835ab1f8e4b23a105605a7"
+  integrity sha512-zJ2mQYM18rEFOudeV4GShTGIQ7RbzA7ozbU9I/XBpm7kqgMywgmylMwXHxZJmkVoYkna9d2pVXVXPdYTP9ej8Q==
+
 from2@^2.1.0:
   version "2.3.0"
   resolved "https://registry.yarnpkg.com/from2/-/from2-2.3.0.tgz#8bfb5502bde4a4d36cfdeea007fcca21d7e382af"