throw error if .next/static cannot be found (#148)

* throw error if .next/static cannot be found

* throw new Error

Co-authored-by: ehmicky <ehmicky@users.noreply.github.com>

Co-authored-by: ehmicky <ehmicky@users.noreply.github.com>

Author: lindsaylevine

lib/steps/copyNextAssets.js

@@ -1,20 +1,20 @@
 const { join } = require("path");
-const { copySync } = require("fs-extra");
+const { copySync, existsSync } = require("fs-extra");
 const { logTitle } = require("../helpers/logger");
 const { NEXT_DIST_DIR } = require("../config");
 
 // Copy the NextJS' static assets from NextJS distDir to Netlify publish folder.
 // These need to be available for NextJS to work.
 const copyNextAssets = (publishPath) => {
+  const staticAssetsPath = join(NEXT_DIST_DIR, "static");
+  if (!existsSync(staticAssetsPath)) {
+    throw new Error("No static assets found in .next dist (aka no /.next/static). Please check your project configuration. Your next.config.js must be one of `serverless` or `experimental-serverless-trace`. Your build command should include `next build`.");
+  }
   logTitle("💼 Copying static NextJS assets to", publishPath);
-  copySync(
-    join(NEXT_DIST_DIR, "static"),
-    join(publishPath, "_next", "static"),
-    {
-      overwrite: false,
-      errorOnExist: true,
-    }
-  );
+  copySync(staticAssetsPath, join(publishPath, "_next", "static"), {
+    overwrite: false,
+    errorOnExist: true,
+  });
 };
 
 module.exports = copyNextAssets;