feat: add Nextjs example and middleware, close #63

Author: bahmutov

Diff for: README.md

@@ -236,6 +236,7 @@ For example, if you want to only include files in the `app` folder, but exclude
 - [rootstrap/react-redux-base](https://github.com/rootstrap/react-redux-base) shows an example with a realistic Webpack config. Instruments the source code using `babel-plugin-istanbul` during tests.
 - [skylock/cypress-angular-coverage-example](https://github.com/skylock/cypress-angular-coverage-example) shows Angular 8 + TypeScript application with instrumentation done using [istanbul-instrumenter-loader](https://github.com/webpack-contrib/istanbul-instrumenter-loader).
 - [bahmutov/testing-react](https://github.com/bahmutov/testing-react) shows how to get code coverage for a React application created using [CRA v3](https://github.com/facebook/create-react-app) without ejecting `react-scripts`.
+- [bahmutov/next-and-cypress-example](https://github.com/bahmutov/next-and-cypress-example) shows how to get backend and fronend coverage for a [Next.js](https://nextjs.org) project. Uses [middleware/nextjs.js](middleware/nextjs.js).
 
 ## Debugging
 

Diff for: middleware/nextjs.js

@@ -0,0 +1,28 @@
+/**
+ * Middleware for returning server-side code coverage
+ * for Next.js API route. To use, create new `pages/api/coverage.js` file
+ * and re-export this default middleware function.
+ *
+  ```
+  // in your pages/api/coverage.js
+  module.exports = require('@cypress/code-coverage/middleware/nextjs')
+  // then add to your cypress.json an environment variable pointing at the API
+  {
+    "baseUrl": "http://localhost:3000",
+    "env": {
+      "codeCoverage": {
+        "url": "/api/__coverage__"
+      }
+    }
+  }
+  ```
+ *
+ * @see https://nextjs.org/docs#api-routes
+ * @see https://github.com/cypress-io/code-coverage
+ */
+module.exports = function returnCodeCoverageNext (req, res) {
+  // only GET is supported
+  res.status(200).json({
+    coverage: global.__coverage__ || null
+  })
+}