fix(instrumenter): don't place mutants inside delete expressions

packages/instrumenter/src/mutant-placers/expression-mutant-placer.ts

@@ -78,7 +78,7 @@ function isCallExpression(path: NodePath): path is NodePath<babel.types.CallExpr
 
 function isValidExpression(path: NodePath<babel.types.Expression>) {
   const parent = path.parentPath;
-  return !isObjectPropertyKey() && !isPartOfChain() && !parent.isTaggedTemplateExpression();
+  return !isObjectPropertyKey() && !isPartOfChain() && !parent.isTaggedTemplateExpression() && !isPartOfDeleteExpression();
 
   /**
    * Determines if the expression is property of an object.
@@ -112,12 +112,22 @@ function isValidExpression(path: NodePath<babel.types.Expression>) {
         (isCallExpression(parent) && parent.node.callee === path.node))
     );
   }
+
+  /**
+   * Determines if the expression is part of a delete expression.
+   * @returns true if the expression is part of a delete expression
+   * @example
+   * delete foo.bar;
+   */
+  function isPartOfDeleteExpression() {
+    return parent.isUnaryExpression() && parent.node.operator === 'delete';
+  }
 }
 
 /**
  * Places the mutants with a conditional expression: `global.activeMutant === 1? mutatedCode : originalCode`;
  */
-export const expressionMutantPlacer: MutantPlacer<babel.types.Expression> = {
+export const expressionMutantPlacer = {
   name: 'expressionMutantPlacer',
   canPlace(path) {
     return path.isExpression() && isValidExpression(path);
@@ -135,4 +145,4 @@ export const expressionMutantPlacer: MutantPlacer<babel.types.Expression> = {
     }
     path.replaceWith(expression);
   },
-};
+} satisfies MutantPlacer<babel.types.Expression>;

packages/instrumenter/test/unit/mutant-placers/expression-mutant-placer.spec.ts

@@ -35,6 +35,20 @@ describe('expressionMutantPlacer', () => {
       expect(expressionMutantPlacer.canPlace(templateLiteral)).false;
     });
 
+    it('should be false when the parent is a delete unary expression', () => {
+      const memberExpression = findNodePath(parseJS('delete myVariable?.[indexer];'), (p) => p.isOptionalMemberExpression());
+      expect(expressionMutantPlacer.canPlace(memberExpression)).false;
+    });
+
+    it('should be true when the parent is a non-delete unary expression', () => {
+      const memberExpression = findNodePath(parseJS('void myVariable[indexer];'), (p) => p.isMemberExpression());
+      const memberExpression2 = findNodePath(parseJS('typeof myVariable[indexer];'), (p) => p.isMemberExpression());
+      const memberExpression3 = findNodePath(parseJS('throw myVariable[indexer];'), (p) => p.isMemberExpression());
+      expect(expressionMutantPlacer.canPlace(memberExpression)).true;
+      expect(expressionMutantPlacer.canPlace(memberExpression2)).true;
+      expect(expressionMutantPlacer.canPlace(memberExpression3)).true;
+    });
+
     describe('object literals', () => {
       it('should be false when the expression is a key', () => {
         // A stringLiteral is considered an expression, while it is not save to place a mutant there!

packages/instrumenter/testResources/instrumenter/optional-chains.ts

@@ -11,4 +11,8 @@ input?.id!.toString();
 
 bar?.baz[0]
 
-state.stats[organization?.organization_id] = action.payload.stats;
+state.stats[organization?.organization_id] = action.payload.stats;
+
+// https://github.com/stryker-mutator/stryker-js/issues/4741
+delete myVariable?.[indexer]; 
+void foo?.();

packages/instrumenter/testResources/instrumenter/optional-chains.ts.out.snap

@@ -53,5 +53,9 @@ const directiveRanges = stryMutAct_9fa48(\\"6\\") ? comments.map(tryParseTSDirec
 const qux = quux(stryMutAct_9fa48(\\"7\\") ? corge.cov() : (stryCov_9fa48(\\"7\\"), corge?.cov()));
 stryMutAct_9fa48(\\"8\\") ? input.id!.toString() : (stryCov_9fa48(\\"8\\"), input?.id!.toString());
 stryMutAct_9fa48(\\"9\\") ? bar.baz[0] : (stryCov_9fa48(\\"9\\"), bar?.baz[0]);
-state.stats[stryMutAct_9fa48(\\"10\\") ? organization.organization_id : (stryCov_9fa48(\\"10\\"), organization?.organization_id)] = action.payload.stats;"
+state.stats[stryMutAct_9fa48(\\"10\\") ? organization.organization_id : (stryCov_9fa48(\\"10\\"), organization?.organization_id)] = action.payload.stats;
+
+// https://github.com/stryker-mutator/stryker-js/issues/4741
+stryMutAct_9fa48(\\"11\\") ? delete myVariable[indexer] : (stryCov_9fa48(\\"11\\"), delete myVariable?.[indexer]);
+void (stryMutAct_9fa48(\\"12\\") ? foo() : (stryCov_9fa48(\\"12\\"), foo?.()));"
 `;