feat: save unit test code coverage from specs

Author: bahmutov

README.md

@@ -32,6 +32,39 @@ If your application is loaded Istanbul-instrumented source code, then the covera
 
 ![Coverage report](images/coverage.jpg)
 
+## Instrument unit tests
+
+If you test your application code directly from `specs` you might want to instrument them and combine unit test code coverage with any end-to-end code coverage (from iframe). You can easily instrument spec files using [babel-plugin-istanbul](https://github.com/istanbuljs/babel-plugin-istanbul) for example. Put the following in `cypress/plugins/index.js` file to use `.babelrc` file
+
+```js
+const browserify = require('@cypress/browserify-preprocessor')
+
+module.exports = (on, config) => {
+  on('task', require('cypress-istanbul/task'))
+
+  // tell Cypress to use .babelrc when bundling spec code
+  const options = browserify.defaultOptions
+  options.browserifyOptions.transform[1][1].babelrc = true
+  on('file:preprocessor', browserify(options))
+}
+```
+
+Install the plugin
+
+```
+npm i -D babel-plugin-istanbul
+```
+
+and set in your `.babelrc` file
+
+```rc
+{
+  "plugins": ["istanbul"]
+}
+```
+
+Now the code coverage from spec files will be combined with end-to-end coverage.
+
 ## Examples
 
 - [Demo battery app](https://github.com/bahmutov/demo-battery-api/tree/bundle) branch "bundle"

support.js

@@ -11,13 +11,35 @@ afterEach(() => {
   // because the entire "window" object is about
   // to be recycled by Cypress before next test
   cy.window().then(win => {
-    if (win.__coverage__) {
-      cy.task('combineCoverage', win.__coverage__)
+    // if application code has been instrumented, the app iframe "window" has an object
+    const applicationSourceCoverage = win.__coverage__
+
+    if (applicationSourceCoverage) {
+      cy.task('combineCoverage', applicationSourceCoverage)
     }
   })
 })
 
 after(() => {
+  const specFolder = Cypress.config('integrationFolder')
+  const supportFolder = Cypress.config('supportFolder')
+
+  // if spec bundle has been instrumented (using Cypress preprocessor)
+  // then we will have unit test coverage
+  // NOTE: spec iframe is NOT reset between the tests, so we can grab
+  // the coverage information only once after all tests have finished
+  const unitTestCoverage = window.__coverage__
+  if (unitTestCoverage) {
+    // remove coverage for the spec files themselves,
+    // only keep "external" application source file coverage
+    const coverage = Cypress._.omitBy(
+      window.__coverage__,
+      (fileCoverage, filename) =>
+        filename.startsWith(specFolder) || filename.startsWith(supportFolder)
+    )
+    cy.task('combineCoverage', coverage)
+  }
+
   // when all tests finish, lets generate the coverage report
   cy.task('coverageReport')
 })