feat: warn if missing code coverage information (#128)

* feat: warn if missing code coverage information

* add note to plugin's log messages

Author: bahmutov

support.js

@@ -5,13 +5,22 @@
  * via "cy.task".
  */
 const sendCoverage = (coverage, pathname = '/') => {
-  cy.log(`Saving code coverage **${pathname}**`)
+  logMessage(`Saving code coverage for **${pathname}**`)
   // stringify coverage object for speed
   cy.task('combineCoverage', JSON.stringify(coverage), {
     log: false
   })
 }
 
+/**
+ * Consistently logs the given string to the Command Log
+ * so the user knows the log message is coming from this plugin.
+ * @param {string} s Message to log.
+ */
+const logMessage = s => {
+  cy.log(`${s} \`[@cypress/code-coverage]\``)
+}
+
 // to disable code coverage commands and save time
 // pass environment variable coverage=false
 //  cypress run --env coverage=false
@@ -21,6 +30,10 @@ if (Cypress.env('coverage') === false) {
 } else {
   let windowCoverageObjects
 
+  const hasE2ECoverage = () => Boolean(windowCoverageObjects.length)
+
+  const hasUnitTestCoverage = () => Boolean(window.__coverage__)
+
   before(() => {
     // we need to reset the coverage when running
     // in the interactive mode, otherwise the counters will
@@ -53,9 +66,26 @@ if (Cypress.env('coverage') === false) {
     windowCoverageObjects.forEach(cover => {
       sendCoverage(cover.coverage, cover.pathname)
     })
+
+    if (!hasE2ECoverage()) {
+      if (hasUnitTestCoverage()) {
+        logMessage(`👉 Only found unit test code coverage.`)
+      } else {
+        logMessage(`
+          ⚠️ Could not find any coverage information in your application
+          by looking at the window coverage object.
+          Did you forget to instrument your application?
+          See [code-coverage#instrument-your-application](https://github.com/cypress-io/code-coverage#instrument-your-application)
+        `)
+      }
+    }
   })
 
   after(() => {
+    // I wish I could fail the tests if there is no code coverage information
+    // but throwing an error here does not fail the test run due to
+    // https://github.com/cypress-io/cypress/issues/2296
+
     // there might be server-side code coverage information
     // we should grab it once after all tests finish
     const baseUrl = Cypress.config('baseUrl') || cy.state('window').origin