Stopped mutating nodes with a declare modifier

packages/typescript/src/TypescriptMutator.ts

@@ -30,6 +30,10 @@ export class TypescriptMutator {
   }
 
   private mutateForNode<T extends ts.Node>(node: T, sourceFile: ts.SourceFile): Mutant[] {
+    if (shouldNodeBeSkipped(node)) {
+      return [];
+    }
+
     const targetMutators = this.mutators.filter(mutator => mutator.guard(node));
     const mutants = flatMap(targetMutators, mutator => mutator.mutate(node, sourceFile));
     node.forEachChild(child => {
@@ -39,3 +43,7 @@ export class TypescriptMutator {
     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 +90,25 @@ describe('TypescriptMutator', () => {
       expect(mutants.filter(mutant => mutant.mutatorName === 'FunctionDeclarationForTest')).lengthOf(4);
     });
 
+    describe('declaration nodes', () => {
+      beforeEach(() => {
+        sut = createSut();
+        file1 = new File(
+          'file1.ts',
+          `declare function notMutated(a: number, b: number);
+          declare module "not-mutated" { }
+
+          function mutated(a: number) { }`);
+      });
+
+      it('should skip nodes with declare keywords', () => {
+        const mutants = sut.mutate([
+          file1,
+        ]);
+        expect(mutants.filter(mutant => mutant.mutatorName === 'SourceFileForTest')).lengthOf(1);
+        expect(mutants.filter(mutant => mutant.mutatorName !== 'SourceFileForTest')).lengthOf(2);
+      });
+    });
+
   });
 });