Better dev server integration

server/middleware/staticAssets.js

@@ -0,0 +1,30 @@
+const fs = require("fs");
+const invariant = require("invariant");
+const createBundle = require("./utils/createBundle");
+
+/**
+ * An express middleware that sets req.bundle from the build
+ * info in the given stats file. Should be used in production.
+ */
+function staticAssets(webpackStatsFile) {
+  let stats;
+  try {
+    stats = JSON.parse(fs.readFileSync(webpackStatsFile, "utf8"));
+  } catch (error) {
+    invariant(
+      false,
+      "staticAssets middleware cannot read the build stats in %s; " +
+        "run `yarn build` before starting the server",
+      webpackStatsFile
+    );
+  }
+
+  const bundle = createBundle(stats);
+
+  return (req, res, next) => {
+    req.bundle = bundle;
+    next();
+  };
+}
+
+module.exports = staticAssets;