fix(babel-transformer): respect top of the file comments/pragma

This means that the stryker mutator hooks are no longer just
prepended to the beginning of file. Instead we also add the comments
which means things like flow should behave correctly

e2e/package.json

@@ -8,6 +8,7 @@
     "@babel/plugin-proposal-class-properties": "~7.8.3",
     "@babel/plugin-proposal-pipeline-operator": "~7.8.3",
     "@babel/preset-env": "~7.8.3",
+    "@babel/preset-flow": "7.12.13",
     "@types/node": "^10.12.18",
     "@types/semver": "~6.2.0",
     "ajv": "~7.0.2",

e2e/test/flow-test-project/.babelrc

@@ -0,0 +1,16 @@
+{
+    "presets": [
+        "@babel/preset-flow",
+        [
+            "@babel/preset-env",
+            {
+                "targets": {
+                    "edge": "16",
+                    "firefox": "57",
+                    "chrome": "62",
+                    "safari": "11"
+                }
+            }
+        ]
+    ]
+}

e2e/test/flow-test-project/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "flow-jest-project",
+  "version": "0.0.0",
+  "private": true,
+  "description": "A module to test a flow project, see https://flow.org/",
+  "scripts": {
+    "test:unit": "jest",
+    "test": "stryker run",
+    "posttest": "mocha --no-config --require ../../tasks/ts-node-register.js verify/*.ts"
+  }
+}

e2e/test/flow-test-project/src/square.js

@@ -0,0 +1,6 @@
+// @flow
+export function square(n: number): number {
+  const result: Array<?number> = [];
+  const cat = new Set<string>();
+  return n * n;
+}

e2e/test/flow-test-project/src/square.spec.js

@@ -0,0 +1,8 @@
+import { square } from './square';
+import { expect } from 'chai';
+
+describe('square', () => {
+  it('should provide 4 when given 2', () => {
+    expect(square(2)).eq(4);
+  });
+});

e2e/test/flow-test-project/stryker.conf.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "../../node_modules/@stryker-mutator/core/schema/stryker-schema.json",
+  "reporters": [
+    "clear-text",
+    "event-recorder"
+  ],
+  "concurrency": 2,
+  "commandRunner": {
+    "command": "npm run test:unit"
+  },
+  "symlinkNodeModules": false,
+  "fileLogLevel": "info"
+}

e2e/test/flow-test-project/verify/.babelrc

@@ -0,0 +1,16 @@
+{
+    "presets": [
+        "@babel/preset-flow",
+        [
+            "@babel/preset-env",
+            {
+                "targets": {
+                    "edge": "16",
+                    "firefox": "57",
+                    "chrome": "62",
+                    "safari": "11"
+                }
+            }
+        ]
+    ]
+}

e2e/test/flow-test-project/verify/verify.ts

@@ -0,0 +1,19 @@
+import { expectMetrics } from '../../../helpers';
+
+describe('After running stryker on jest-react project', () => {
+  it('should report expected scores', async () => {
+    await expectMetrics({
+      killed: 2,
+      ignored: 0,
+      survived: 1,
+      mutationScore: 66.67,
+    });
+    /*
+    -----------|---------|----------|-----------|------------|----------|---------|
+    File       | % score | # killed | # timeout | # survived | # no cov | # error |
+    -----------|---------|----------|-----------|------------|----------|---------|
+    All files  |   66.67 |        2 |         0 |          1 |        0 |       0 |
+     square.js |   66.67 |        2 |         0 |          1 |        0 |       0 |
+    -----------|---------|----------|-----------|------------|----------|---------|*/
+  });
+});

packages/instrumenter/src/transformers/babel-transformer.ts

@@ -36,6 +36,11 @@ export const transformBabel: AstTransformer<AstFormat.JS | AstFormat.TS> = ({ ro
     },
   });
   if (mutantCollector.hasPlacedMutants(originFileName)) {
+    const innerComments = root.program.innerComments ?? [];
+    const leadingComments = root.program.body[0]?.leadingComments ?? [];
+    if (Array.isArray(leadingComments)) {
+      instrumentationBabelHeader[0].leadingComments = [...innerComments, ...leadingComments];
+    }
     root.program.body.unshift(...instrumentationBabelHeader);
   }
 };

packages/instrumenter/test/unit/transformers/babel-transformer.spec.ts

@@ -80,10 +80,26 @@ describe('babel-transformer', () => {
     expect(mutantCollectorMock.markMutantsAsPlaced).calledWith([mutant]);
   });
 
-  it('should add the global stuff on top', () => {
-    const ast = createJSAst({ rawContent: 'foo' });
+  it('should add the global stuff on top but after comments that are followed by newline', () => {
+    const ast = createJSAst({ rawContent: '// @flow\n// another comment\n\nconst foo="cat"' });
     mutantCollectorMock.hasPlacedMutants.returns(true);
     transformBabel(ast, mutantCollectorMock, context);
+
+    expect(ast.root.program.body[0].leadingComments![0].value).eq(' @flow');
+    expect(ast.root.program.body[0].leadingComments![1].value).eq(' another comment');
+
+    for (let i = 0; i < instrumentationBabelHeader.length; i++) {
+      expect(ast.root.program.body[i]).eq(instrumentationBabelHeader[i]);
+    }
+  });
+  it('should add the global stuff on top but after comments that are followed by a statement', () => {
+    const ast = createJSAst({ rawContent: '// @flow\n// another comment\nconst foo="cat"' });
+    mutantCollectorMock.hasPlacedMutants.returns(true);
+    transformBabel(ast, mutantCollectorMock, context);
+
+    expect(ast.root.program.body[0].leadingComments![0].value).eq(' @flow');
+    expect(ast.root.program.body[0].leadingComments![1].value).eq(' another comment');
+
     for (let i = 0; i < instrumentationBabelHeader.length; i++) {
       expect(ast.root.program.body[i]).eq(instrumentationBabelHeader[i]);
     }