up-to-date check script (mdn#1095)

* up-to-date check script

* feedbacked

package.json

@@ -7,7 +7,8 @@
     "node": ">=12.0.0"
   },
   "scripts": {
-    "start": "env-cmd --silent cross-env CONTENT_ROOT=files REACT_APP_DISABLE_AUTH=true node node_modules/@mdn/yari/server",
+    "up-to-date-check": "node scripts/up-to-date-check.js",
+    "start": "yarn up-to-date-check && env-cmd --silent cross-env CONTENT_ROOT=files REACT_APP_DISABLE_AUTH=true node node_modules/@mdn/yari/server",
     "filecheck": "env-cmd --silent cross-env CONTENT_ROOT=files node node_modules/@mdn/yari/filecheck/cli.js --cwd=$(pwd)",
     "content": "env-cmd --silent cross-env CONTENT_ROOT=files node node_modules/@mdn/yari/tool/cli.js"
   },

scripts/up-to-date-check.js

@@ -0,0 +1,15 @@
+const yariPackageJson = require("@mdn/yari/package.json");
+const thisPackageJson = require("../package.json");
+
+const availableYariVersion = thisPackageJson.dependencies["@mdn/yari"];
+const installedYariVersion = yariPackageJson.version;
+
+if (installedYariVersion < availableYariVersion) {
+  console.log(
+    `The installed version of Yari (${installedYariVersion}) is older than ` +
+      `the version referenced in package.json (${availableYariVersion}). ` +
+      `Please make sure your main git branch is up-to-date with ` +
+      `https://github.com/mdn/content/, then run yarn install to ` +
+      `install the latest Yari.`
+  );
+}