fix(ArrowFunction mutator): don't mutate () => undefined (#2313)

stryker-mutator · Aug 3, 2020 · 310145e · 310145e
1 parent 0010741
commit 310145e
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Showing 2 changed files with 10 additions and 2 deletions.
diff --git a/packages/instrumenter/src/mutators/arrow-function-mutator.ts b/packages/instrumenter/src/mutators/arrow-function-mutator.ts
@@ -8,9 +8,13 @@ import { NodeMutator } from '.';
 export class ArrowFunctionMutator implements NodeMutator {
   public name = 'ArrowFunction';
 
+  private readonly undefinedIdentifierName = 'undefined';
+
   public mutate(path: NodePath): NodeMutation[] {
-    return path.isArrowFunctionExpression() && !types.isBlockStatement(path.node.body)
-      ? [{ original: path.node, replacement: types.arrowFunctionExpression([], types.identifier('undefined')) }]
+    return path.isArrowFunctionExpression() &&
+      !types.isBlockStatement(path.node.body) &&
+      !(types.isIdentifier(path.node.body) && path.node.body.name === this.undefinedIdentifierName)
+      ? [{ original: path.node, replacement: types.arrowFunctionExpression([], types.identifier(this.undefinedIdentifierName)) }]
       : [];
   }
 }
diff --git a/packages/instrumenter/test/unit/mutators/arrow-function-mutator.spec.ts b/packages/instrumenter/test/unit/mutators/arrow-function-mutator.spec.ts
@@ -20,4 +20,8 @@ describe(ArrowFunctionMutator.name, () => {
   it('should not mutate an anonymous function with a block as a body', () => {
     expectJSMutation(sut, 'const b = () => { return 4; }');
   });
+
+  it('should not mutate an anonymous function with undefined as a body', () => {
+    expectJSMutation(sut, 'const b = () => undefined');
+  });
 });