Incorrect empty file coverage (#7388)

CHANGELOG.md

@@ -45,6 +45,7 @@
 
 ### Fixes
 
+- `[jest-cli]` Fix empty coverage data for untested files ([#7388](https://github.com/facebook/jest/pull/7388))
 - `[jest-cli]` [**BREAKING**] Do not use `text-summary` coverage reporter by default if other reporters are configured ([#7058](https://github.com/facebook/jest/pull/7058))
 - `[jest-mock]` [**BREAKING**] Fix bugs with mock/spy result tracking of recursive functions ([#6381](https://github.com/facebook/jest/pull/6381))
 - `[jest-haste-map]` [**BREAKING**] Recover files correctly after haste name collisions are fixed ([#7329](https://github.com/facebook/jest/pull/7329))

packages/jest-cli/src/__tests__/__snapshots__/generateEmptyCoverage.test.js.snap

@@ -75,7 +75,7 @@ Object {
       "name": "(anonymous_0)",
     },
   },
-  "path": "/sum.js",
+  "path": "/tmp/sum.js",
   "s": Object {
     "0": 0,
     "1": 0,

packages/jest-cli/src/__tests__/generateEmptyCoverage.test.js

@@ -8,9 +8,12 @@
  */
 'use strict';
 
+import istanbulCoverage from 'istanbul-lib-coverage';
+import libSourceMaps from 'istanbul-lib-source-maps';
 import generateEmptyCoverage from '../generateEmptyCoverage';
 import Runtime from 'jest-runtime';
 
+const path = require('path');
 const os = require('os');
 const {makeGlobalConfig, makeProjectConfig} = require('../../../../TestUtils');
 
@@ -32,14 +35,32 @@ module.exports = {
 };`;
 
 it('generates an empty coverage object for a file without running it', () => {
+  const coverageMap = istanbulCoverage.createCoverageMap({});
+  const sourceMapStore = libSourceMaps.createSourceMapStore();
+  const rootDir = '/tmp';
+  const filepath = path.join(rootDir, './sum.js');
+
   const emptyCoverage = generateEmptyCoverage(
     src,
-    '/sum.js',
+    filepath,
     makeGlobalConfig(),
     makeProjectConfig({
       cacheDirectory: os.tmpdir(),
-      rootDir: os.tmpdir(),
+      rootDir,
+      transform: [['^.+\\.js$', require.resolve('babel-jest')]],
     }),
   );
-  expect(emptyCoverage && emptyCoverage.coverage).toMatchSnapshot();
+
+  expect(typeof emptyCoverage).toBe('object');
+
+  let coverage = emptyCoverage && emptyCoverage.coverage;
+
+  if (emptyCoverage && emptyCoverage.sourceMapPath) {
+    coverageMap.addFileCoverage(emptyCoverage.coverage);
+    sourceMapStore.registerURL(filepath, emptyCoverage.sourceMapPath);
+
+    coverage = sourceMapStore.transformCoverage(coverageMap).map;
+  }
+
+  expect(coverage).toMatchSnapshot();
 });

packages/jest-cli/src/generateEmptyCoverage.js

@@ -9,14 +9,17 @@
 
 import type {GlobalConfig, ProjectConfig, Path} from 'types/Config';
 
-import {createInstrumenter} from 'istanbul-lib-instrument';
+import {readInitialCoverage} from 'istanbul-lib-instrument';
+import {classes} from 'istanbul-lib-coverage';
 import Runtime from 'jest-runtime';
 
 export type CoverageWorkerResult = {|
   coverage: any,
   sourceMapPath: ?string,
 |};
 
+const {FileCoverage} = classes;
+
 export default function(
   source: string,
   filename: Path,
@@ -29,16 +32,15 @@ export default function(
     collectCoverageOnlyFrom: globalConfig.collectCoverageOnlyFrom,
   };
   if (Runtime.shouldInstrument(filename, coverageOptions, config)) {
-    // Transform file without instrumentation first, to make sure produced
-    // source code is ES6 (no flowtypes etc.) and can be instrumented
-    const transformResult = new Runtime.ScriptTransformer(
+    // Transform file with instrumentation to make sure initial coverage data is well mapped to original code.
+    const {code, mapCoverage, sourceMapPath} = new Runtime.ScriptTransformer(
       config,
-    ).transformSource(filename, source, false);
-    const instrumenter = createInstrumenter();
-    instrumenter.instrumentSync(transformResult.code, filename);
+    ).transformSource(filename, source, true);
+    const extracted = readInitialCoverage(code);
+
     return {
-      coverage: instrumenter.fileCoverage,
-      sourceMapPath: transformResult.sourceMapPath,
+      coverage: new FileCoverage(extracted.coverageData),
+      sourceMapPath: mapCoverage ? sourceMapPath : null,
     };
   } else {
     return null;