feat: add option to expect the backend code coverage only (#373)

`expectBackendCoverageOnly: true`

Author: bahmutov

README.md

@@ -207,6 +207,29 @@ if (global.__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.
 
+### expectBackendCoverageOnly
+
+If there is NO frontend code coverage, and you want to only collect the backend code coverage using Cypress tests, set `expectBackendCoverageOnly: true` in `cypress.json` file. Otherwise Cypress complains that it cannot find the frontend code coverage.
+
+Default:
+
+![No frontend code coverage warning](./images/warning.png)
+
+After:
+
+```json
+{
+  "env": {
+    "codeCoverage": {
+      "url": "http://localhost:3003/__coverage__",
+      "expectBackendCoverageOnly": true
+    }
+  }
+}
+```
+
+![Cypress knows to expect the backend code coverage only](./images/expect-backend.png)
+
 ## Custom report folder
 
 You can specify custom report folder by adding `nyc` object to the `package.json` file. For example to save reports to `cypress-coverage` folder, use:

examples/backend/cypress.json

@@ -3,7 +3,8 @@
   "baseUrl": "http://localhost:3003",
   "env": {
     "codeCoverage": {
-      "url": "http://localhost:3003/__coverage__"
+      "url": "http://localhost:3003/__coverage__",
+      "expectBackendCoverageOnly": true
     }
   }
 }

examples/backend/package.json

@@ -2,6 +2,7 @@
   "name": "example-backend",
   "description": "Code coverage for backend",
   "devDependencies": {},
+  "private": true,
   "scripts": {
     "start": "../../node_modules/.bin/nyc --silent node server/server",
     "cy:open": "../../node_modules/.bin/cypress open",

images/expect-backend.png

@@ -134,12 +134,19 @@ const registerHooks = () => {
       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)
-        `)
+        const expectBackendCoverageOnly = Cypress._.get(
+          Cypress.env('codeCoverage'),
+          'expectBackendCoverageOnly',
+          false
+        )
+        if (!expectBackendCoverageOnly) {
+          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)
+          `)
+        }
       }
     }
   })
@@ -179,7 +186,19 @@ const registerHooks = () => {
           if (!coverage) {
             // we did not get code coverage - this is the
             // original failed request
-            return
+            const expectBackendCoverageOnly = Cypress._.get(
+              Cypress.env('codeCoverage'),
+              'expectBackendCoverageOnly',
+              false
+            )
+            if (expectBackendCoverageOnly) {
+              throw new Error(
+                `Expected to collect backend code coverage from ${url}`
+              )
+            } else {
+              // we did not really expect to collect the backend code coverage
+              return
+            }
           }
           sendCoverage(coverage, 'backend')
         })