fix(typescript): don't mutate declare statements (#1458)

Stop mutating `declare` expressions. Like `declare function notMutated(a: number, b: number);`

packages/typescript/src/TypescriptMutator.ts

@@ -30,12 +30,20 @@ export class TypescriptMutator {
   }
 
   private mutateForNode<T extends ts.Node>(node: T, sourceFile: ts.SourceFile): Mutant[] {
-    const targetMutators = this.mutators.filter(mutator => mutator.guard(node));
-    const mutants = flatMap(targetMutators, mutator => mutator.mutate(node, sourceFile));
-    node.forEachChild(child => {
-      // It is important that forEachChild does not return a true, otherwise node visiting is halted!
-      mutants.push(... this.mutateForNode(child, sourceFile));
-    });
-    return mutants;
+    if (shouldNodeBeSkipped(node)) {
+      return [];
+    } else {
+      const targetMutators = this.mutators.filter(mutator => mutator.guard(node));
+      const mutants = flatMap(targetMutators, mutator => mutator.mutate(node, sourceFile));
+      node.forEachChild(child => {
+        // It is important that forEachChild does not return a true, otherwise node visiting is halted!
+        mutants.push(... this.mutateForNode(child, sourceFile));
+      });
+      return mutants;
+    }
   }
 }
+
+const shouldNodeBeSkipped = (node: ts.Node): boolean => {
+  return node.modifiers !== undefined && node.modifiers.some(modifier => modifier.kind === ts.SyntaxKind.DeclareKeyword);
+};

packages/typescript/test/unit/TypescriptMutator.spec.ts

@@ -90,5 +90,67 @@ describe('TypescriptMutator', () => {
       expect(mutants.filter(mutant => mutant.mutatorName === 'FunctionDeclarationForTest')).lengthOf(4);
     });
 
+    describe('declaration nodes', () => {
+      beforeEach(() => {
+        sut = createSut();
+      });
+
+      it('should not skip a node when it does not have a declare modifier', () => {
+        // Arrange
+        file1 = new File(
+          'file1.ts',
+          `function mutated(a: number, b: number);`);
+
+        // Act
+        const mutants = sut.mutate([
+          file1,
+        ]);
+
+        // Assert
+        expect(mutants).to.deep.equal([
+          {
+            fileName: 'file1.ts',
+            mutatorName: 'SourceFileForTest',
+            range: [0, 39],
+            replacement: '"stryker was here"'
+          },
+          {
+            fileName: 'file1.ts',
+            mutatorName: 'FunctionDeclarationForTest',
+            range: [0, 39],
+            replacement: '// Function declaration removed'
+          },
+          {
+            fileName: 'file1.ts',
+            mutatorName: 'FunctionDeclarationForTest',
+            range: [0, 39],
+            replacement: 'changedToOtherCall()'
+          }
+        ]);
+      });
+
+      it('should skip a node when it has a declare modifier', () => {
+        // Arrange
+        file1 = new File(
+          'file1.ts',
+          `declare function notMutated(a: number, b: number);`);
+
+        // Act
+        const mutants = sut.mutate([
+          file1,
+        ]);
+
+        // Assert
+        expect(mutants).to.deep.equal([
+          {
+            fileName: 'file1.ts',
+            mutatorName: 'SourceFileForTest',
+            range: [0, 50],
+            replacement: '"stryker was here"',
+          }
+        ]);
+      });
+    });
+
   });
 });