Support for code coverage instrumentation in the app code

Created by: Enselic

Just like you can say npm test -- --coverage I think it would be useful to be able to say npm start -- --coverage or npm run build -- --coverage to instrument the app code for code coverage. You could then get code coverage information from end-to-end tests (which IMO are the most useful kind of tests). Here'a simple step-by-step demo to show what I mean:

1. create-react-app coverage-demo
2. cd coverage-demo
3. npm install --save-dev chromedriver istanbul-lib-coverage selenium-webdriver
4. To src/App.test.js, add

import { Builder } from "selenium-webdriver";
import libCoverage from "istanbul-lib-coverage";

it("report code coverage after end-to-end testing", async () => {
  // Go to site
  const builder = new Builder();
  const driver = builder.forBrowser("chrome").build();
  await driver.get("localhost:3000");

  // A real test would do more here of course

  // Collect coverage data and report it
  const __coverage__ = await driver.executeScript("return __coverage__;");
  const map = libCoverage.createCoverageMap(__coverage__);
  let result = "";
  map.files().forEach(function(f) {
    var fc = map.fileCoverageFor(f);
    result += `${fc.computeSimpleTotals("s").pct + "%"} for ${f}\n`
  });
  console.log(result);

  await driver.quit();
});

5. To const plugins in node_modules/babel-preset-react-app/index.js, add the following item last to the array

// TODO: Enable only when '--coverage' is passed to 'npm start/build'
require.resolve('babel-plugin-istanbul'),

6. npm start
7. npm test # in another terminal

Expected output:

100% for /home/martinno/src/coverage-demo/src/App.js
100% for /home/martinno/src/coverage-demo/src/index.js
6.9% for /home/martinno/src/coverage-demo/src/registerServiceWorker.js

The coverage data can of course be processed in more useful ways. In my own app I write the json data to files and then use istanbul report --root coverage-data html to get a nice HTML presentation of coverage data.

Would you be open to a contribution that did step 5 properly?