feat: add server-side middleware and merge the backend coverage with frontend

.vscode/settings.json

@@ -0,0 +1,3 @@
+{
+  "workbench.colorTheme": "Tomorrow Night Blue"
+}

README.md

@@ -63,10 +63,65 @@ module.exports = (on, config) => {
 
 Now the code coverage from spec files will be combined with end-to-end coverage.
 
+## Instrument backend code
+
+You can also instrument your server-side code and produce combined coverage report that covers both the backend and frontend code.
+
+1. Run the server code with instrumentation. The simplest way is to use [nyc](https://github.com/istanbuljs/nyc). If normally you run `node src/server` then to run instrumented version you can do `nyc --silent node src/server`.
+2. Add an endpoint that returns collected coverage. If you are using Express, you can simply do
+
+```js
+const express = require('express')
+const app = express()
+require('@cypress/code-coverage/middleware')(app)
+```
+
+**Tip:** you can register the endpoint only if there is global code coverage object, and you can exclude the middleware code from the coverage numbers
+
+```js
+// https://github.com/gotwarlost/istanbul/blob/master/ignoring-code-for-coverage.md
+/* istanbul ignore next */
+if (global.__coverage__) {
+  require('@cypress/code-coverage/middleware')(app)
+}
+```
+
+If you use Hapi server, define the endpoint yourself and return the object
+
+```js
+// https://github.com/gotwarlost/istanbul/blob/master/ignoring-code-for-coverage.md
+/* istanbul ignore next */
+if (global.__coverage__) {
+  // https://hapijs.com/tutorials/routing?lang=en_US
+  server.route({
+    method: 'GET',
+    path: '/__coverage__',
+    handler () {
+      return { coverage: global.__coverage__ }
+    }
+  })
+}
+```
+
+3. Save the API coverage endpoint in `cypress.json` file to let the plugin know where to call to receive the code coverage data from the server. Place it in `env.codeCoverage` object:
+
+```json
+{
+  "env": {
+    "codeCoverage": {
+      "url": "http://localhost:3000/__coverage__"
+    }
+  }
+}
+```
+
+That should be enough - the code coverage from the server will be requested at the end of the test run and merged with the client-side code coverage, producing a combined report
+
 ## Examples
 
 - [Cypress code coverage guide](http://on.cypress.io/code-coverage)
 - [cypress-example-todomvc-redux](https://github.com/cypress-io/cypress-example-todomvc-redux)
+- Full frontend + backend code coverage in [bahmutov/realworld](https://github.com/bahmutov/realworld) repo
 - Read ["Code Coverage by Parcel Bundler"](https://glebbahmutov.com/blog/code-coverage-by-parcel/) blog post
 - Read ["Combined End-to-end and Unit Test Coverage"](https://glebbahmutov.com/blog/combined-end-to-end-and-unit-test-coverage/)
 

middleware.js

@@ -0,0 +1,9 @@
+module.exports = app => {
+  // expose "GET __coverage__" endpoint that just returns
+  // global coverage information (if the application has been instrumented)
+  app.get('/__coverage__', (req, res) => {
+    res.json({
+      coverage: global.__coverage__ || null
+    })
+  })
+}

support.js

@@ -21,6 +21,25 @@ afterEach(() => {
 })
 
 after(() => {
+  // there might be server-side code coverage information
+  // we should grab it once after all tests finish
+  const url = Cypress._.get(Cypress.env('codeCoverage'), 'url', '/__coverage__')
+  cy.request({
+    url,
+    log: false,
+    failOnStatusCode: false
+  })
+    .then(r => Cypress._.get(r, 'body.coverage', null), { log: false })
+    .then(coverage => {
+      if (!coverage) {
+        // we did not get code coverage - this is the
+        // original failed request
+        return
+      }
+      cy.task('combineCoverage', coverage)
+    })
+
+  // collect and merge frontend coverage
   const specFolder = Cypress.config('integrationFolder')
   const supportFolder = Cypress.config('supportFolder')