Produce source maps when instrumenting code

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixes
 
 - `[jest-snapshot]` Downgrade semver to v6 to support node 8 ([#9451](https://github.com/facebook/jest/pull/9451))
+- `[jest-transform]` Correct sourcemap behavior for transformed and instrumented code ([#9460](https://github.com/facebook/jest/pull/9460))
 
 ### Chore & Maintenance
 

packages/jest-transform/src/ScriptTransformer.ts

@@ -191,8 +191,12 @@ export default class ScriptTransformer {
     return transform;
   }
 
-  private _instrumentFile(filename: Config.Path, content: string): string {
-    const result = babelTransform(content, {
+  private _instrumentFile(
+    filename: Config.Path,
+    input: TransformedSource,
+    canMapToInput: boolean,
+  ): TransformedSource {
+    const result = babelTransform(input.code, {
       auxiliaryCommentBefore: ' istanbul ignore next ',
       babelrc: false,
       caller: {
@@ -209,21 +213,27 @@ export default class ScriptTransformer {
             // files outside `cwd` will not be instrumented
             cwd: this._config.rootDir,
             exclude: [],
+            // Needed for correct coverage as soon as we start storing a source map of the instrumented code
+            inputSourceMap: input.map,
             useInlineSourceMaps: false,
           },
         ],
       ],
+      /**
+       * It's necessary to be able to map back to original source from the instrumented code.
+       * The inline map is needed for debugging functionality, and exposing it as a separate file is needed
+       * for mapping stack traces. It's convenient to use 'both' here and avoid extracting the source map.
+       *
+       * Previous behavior of emitting no map when we can't map back to original source is preserved.
+       */
+      sourceMaps: canMapToInput ? 'both' : false,
     });
 
-    if (result) {
-      const {code} = result;
-
-      if (code) {
-        return code;
-      }
+    if (result && result.code) {
+      return result as TransformResult;
     }
 
-    return content;
+    return {code: input.code};
   }
 
   private _getRealPath(filepath: Config.Path): Config.Path {
@@ -312,17 +322,36 @@ export default class ScriptTransformer {
       }
     }
 
+    // Apply instrumentation to the code if necessary, keeping the instrumented code and new map
+    let map = transformed.map;
     if (!transformWillInstrument && instrument) {
-      code = this._instrumentFile(filename, transformed.code);
+      /**
+       * We can map the original source code to the instrumented code ONLY if
+       * - the process of transforming the code produced a source map e.g. ts-jest
+       * - we did not transform the source code
+       *
+       * Otherwise we cannot make any statements about how the instrumented code corresponds to the original code,
+       * and we should NOT emit any source maps
+       *
+       */
+      const shouldEmitSourceMaps = (!!transform && !!map) || !transform;
+      const instrumented = this._instrumentFile(
+        filename,
+        transformed,
+        shouldEmitSourceMaps,
+      );
+      code = instrumented.code;
+
+      if (instrumented.map) {
+        map = instrumented.map;
+      }
     } else {
       code = transformed.code;
     }
 
-    if (transformed.map) {
+    if (map) {
       const sourceMapContent =
-        typeof transformed.map === 'string'
-          ? transformed.map
-          : JSON.stringify(transformed.map);
+        typeof map === 'string' ? map : JSON.stringify(map);
       writeCacheFile(sourceMapPath, sourceMapContent);
     } else {
       sourceMapPath = null;

packages/jest-transform/src/__tests__/__snapshots__/script_transformer.test.js.snap

@@ -78,7 +78,7 @@ exports[`ScriptTransformer transforms a file properly 1`] = `
 /* istanbul ignore next */
 function cov_25u22311x4() {
   var path = "/fruits/banana.js";
-  var hash = "4be0f6184160be573fc43f7c2a5877c28b7ce249";
+  var hash = "3f8e915bed83285455a8a16aa04dc0cf5242d755";
   var global = new Function("return this")();
   var gcv = "__coverage__";
   var coverageData = {
@@ -102,8 +102,9 @@ function cov_25u22311x4() {
     },
     f: {},
     b: {},
+    inputSourceMap: null,
     _coverageSchema: "1a1c01bbd47fc00a2c39e90264f33305004495a9",
-    hash: "4be0f6184160be573fc43f7c2a5877c28b7ce249"
+    hash: "3f8e915bed83285455a8a16aa04dc0cf5242d755"
   };
   var coverage = global[gcv] || (global[gcv] = {});
 
@@ -122,13 +123,14 @@ function cov_25u22311x4() {
 
 cov_25u22311x4().s[0]++;
 module.exports = "banana";
+//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImJhbmFuYS5qcyJdLCJuYW1lcyI6WyJtb2R1bGUiLCJleHBvcnRzIl0sIm1hcHBpbmdzIjoiOzs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7OztBQUFBQSxNQUFNLENBQUNDLE9BQVAsR0FBaUIsUUFBakIiLCJzb3VyY2VzQ29udGVudCI6WyJtb2R1bGUuZXhwb3J0cyA9IFwiYmFuYW5hXCI7Il19
 `;
 
 exports[`ScriptTransformer transforms a file properly 2`] = `
 /* istanbul ignore next */
 function cov_23yvu8etmu() {
   var path = "/fruits/kiwi.js";
-  var hash = "7705dd5fcfbc884dcea7062944cfb8cc5d141d1a";
+  var hash = "8b5afd38d79008f13ebc229b89ef82b12ee9447a";
   var global = new Function("return this")();
   var gcv = "__coverage__";
   var coverageData = {
@@ -190,8 +192,9 @@ function cov_23yvu8etmu() {
       "0": 0
     },
     b: {},
+    inputSourceMap: null,
     _coverageSchema: "1a1c01bbd47fc00a2c39e90264f33305004495a9",
-    hash: "7705dd5fcfbc884dcea7062944cfb8cc5d141d1a"
+    hash: "8b5afd38d79008f13ebc229b89ef82b12ee9447a"
   };
   var coverage = global[gcv] || (global[gcv] = {});
 
@@ -216,6 +219,7 @@ module.exports = () => {
   cov_23yvu8etmu().s[1]++;
   return "kiwi";
 };
+//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJzb3VyY2VzIjpbImtpd2kuanMiXSwibmFtZXMiOlsibW9kdWxlIiwiZXhwb3J0cyJdLCJtYXBwaW5ncyI6Ijs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7QUFBQUEsTUFBTSxDQUFDQyxPQUFQLEdBQWlCLE1BQU07QUFBQTtBQUFBO0FBQUE7QUFBQTtBQUFNLENBQTdCIiwic291cmNlc0NvbnRlbnQiOlsibW9kdWxlLmV4cG9ydHMgPSAoKSA9PiBcImtpd2lcIjsiXX0=
 `;
 
 exports[`ScriptTransformer uses multiple preprocessors 1`] = `

packages/jest-transform/src/__tests__/script_transformer.test.js

@@ -400,9 +400,7 @@ describe('ScriptTransformer', () => {
 
     const result = scriptTransformer.transform(
       '/fruits/banana.js',
-      makeGlobalConfig({
-        collectCoverage: true,
-      }),
+      makeGlobalConfig(),
     );
     expect(result.sourceMapPath).toEqual(expect.any(String));
     const mapStr = JSON.stringify(map);
@@ -433,9 +431,7 @@ describe('ScriptTransformer', () => {
 
     const result = scriptTransformer.transform(
       '/fruits/banana.js',
-      makeGlobalConfig({
-        collectCoverage: true,
-      }),
+      makeGlobalConfig(),
     );
     expect(result.sourceMapPath).toEqual(expect.any(String));
     expect(writeFileAtomic.sync).toBeCalledTimes(2);
@@ -504,9 +500,7 @@ describe('ScriptTransformer', () => {
 
     const result = scriptTransformer.transform(
       '/fruits/banana.js',
-      makeGlobalConfig({
-        collectCoverage: true,
-      }),
+      makeGlobalConfig(),
     );
     expect(result.sourceMapPath).toEqual(expect.any(String));
     expect(writeFileAtomic.sync).toBeCalledTimes(2);
@@ -541,6 +535,94 @@ describe('ScriptTransformer', () => {
     expect(writeFileAtomic.sync).toHaveBeenCalledTimes(1);
   });
 
+  it('should write a source map for the instrumented file when transformed', () => {
+    const transformerConfig = {
+      ...config,
+      transform: [['^.+\\.js$', 'preprocessor-with-sourcemaps']],
+    };
+    const scriptTransformer = new ScriptTransformer(transformerConfig);
+
+    const map = {
+      mappings: ';AAAA',
+      version: 3,
+    };
+
+    // A map from the original source to the instrumented output
+    /* eslint-disable sort-keys */
+    const instrumentedCodeMap = {
+      version: 3,
+      sources: ['banana.js'],
+      names: ['content'],
+      mappings: ';;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAAA,OAAO',
+      sourcesContent: ['content'],
+    };
+    /* eslint-enable */
+
+    require('preprocessor-with-sourcemaps').process.mockReturnValue({
+      code: 'content',
+      map,
+    });
+
+    const result = scriptTransformer.transform(
+      '/fruits/banana.js',
+      makeGlobalConfig({
+        collectCoverage: true,
+      }),
+    );
+    expect(result.sourceMapPath).toEqual(expect.any(String));
+    expect(writeFileAtomic.sync).toBeCalledTimes(2);
+    expect(writeFileAtomic.sync).toBeCalledWith(
+      result.sourceMapPath,
+      JSON.stringify(instrumentedCodeMap),
+      {
+        encoding: 'utf8',
+      },
+    );
+
+    // Inline source map allows debugging of original source when running instrumented code
+    expect(result.code).toContain('//# sourceMappingURL');
+  });
+
+  it('should write a source map for the instrumented file when not transformed', () => {
+    const scriptTransformer = new ScriptTransformer(config);
+
+    // A map from the original source to the instrumented output
+    /* eslint-disable sort-keys */
+    const instrumentedCodeMap = {
+      version: 3,
+      sources: ['banana.js'],
+      names: ['module', 'exports'],
+      mappings:
+        ';;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAAA,MAAM,CAACC,OAAP,GAAiB,QAAjB',
+      sourcesContent: ['module.exports = "banana";'],
+    };
+    /* eslint-enable */
+
+    require('preprocessor-with-sourcemaps').process.mockReturnValue({
+      code: 'content',
+      map: null,
+    });
+
+    const result = scriptTransformer.transform(
+      '/fruits/banana.js',
+      makeGlobalConfig({
+        collectCoverage: true,
+      }),
+    );
+    expect(result.sourceMapPath).toEqual(expect.any(String));
+    expect(writeFileAtomic.sync).toBeCalledTimes(2);
+    expect(writeFileAtomic.sync).toBeCalledWith(
+      result.sourceMapPath,
+      JSON.stringify(instrumentedCodeMap),
+      {
+        encoding: 'utf8',
+      },
+    );
+
+    // Inline source map allows debugging of original source when running instrumented code
+    expect(result.code).toContain('//# sourceMappingURL');
+  });
+
   it('passes expected transform options to getCacheKey', () => {
     config = {...config, transform: [['^.+\\.js$', 'test_preprocessor']]};
     const scriptTransformer = new ScriptTransformer(config);