feat(mutators): Implement missing AssignmentOperatorMutator (#3203)

Add the [AssignmentOperatorMutator](https://stryker-mutator.io/docs/mutation-testing-elements/supported-mutators/#assignment-expression)

| Original | Mutated |
| -------- | ------- |
| `+=`     | `-=`    |
| `-=`     | `+=`    |
| `*=`     | `/=`    |
| `/=`     | `*=`    |
| `%=`     | `*=`    |
| `<<=`    | `>>=`   |
| `>>=`    | `<<=`   |
| `&=`     | `\|=`   |
| `|=`     | `&=`    |
| `&&=`    | `||=`   |
|  `||=`   | `&&=`   |
|  `??=`   | `&&=`   |

e2e/test/babel-transpiling/stryker.conf.js

@@ -6,7 +6,7 @@ module.exports = {
   coverageAnalysis: 'off',
   buildCommand: 'npm run build',
   timeoutMS: 60000,
-  reporters: ['clear-text', 'html', 'event-recorder', 'progress'],
+  reporters: ['clear-text', 'html', 'json', 'progress'],
   concurrency: 1,
   logLevel: 'info',
   plugins: [

e2e/test/babel-transpiling/verify/verify.ts

@@ -1,24 +1,15 @@
-import { expectMetricsResult, produceMetrics } from '../../../helpers';
+import { expectMetricsJson } from '../../../helpers';
 
 describe('Verify stryker has ran correctly', () => {
-
   it('should report expected score', async () => {
     // File       | % score | # killed | # timeout | # survived | # no cov | # error |
-    // All files  |   58.54 |       24 |         0 |         17 |        0 |       1 |
-    await expectMetricsResult({
-      metrics: produceMetrics({
-        killed: 24,
-        mutationScore: 55.81,
-        mutationScoreBasedOnCoveredCode: 55.81,
-        runtimeErrors: 1,
-        survived: 19,
-        totalCovered: 43,
-        totalDetected: 24,
-        totalInvalid: 1,
-        totalMutants: 44,
-        totalUndetected: 19,
-        totalValid: 43
-      })
+    // All files  |   57.45 |       27 |         0 |         20 |        0 |       1 |
+    await expectMetricsJson({
+      killed: 27,
+      mutationScore: 57.45,
+      runtimeErrors: 1,
+      survived: 20,
+      noCoverage: 0,
     });
   });
 });

packages/instrumenter/src/mutators/assignment-operator-mutator.ts

@@ -0,0 +1,47 @@
+import * as types from '@babel/types';
+
+import { NodeMutator } from '.';
+
+enum AssignmentOperators {
+  '+=' = '-=',
+  '-=' = '+=',
+  '*=' = '/=',
+  '/=' = '*=',
+  '%=' = '*=',
+  '<<=' = '>>=',
+  '>>=' = '<<=',
+  '&=' = '|=',
+  '|=' = '&=',
+  '&&=' = '||=',
+  '||=' = '&&=',
+  '??=' = '&&=',
+}
+
+const stringTypes = Object.freeze(['StringLiteral', 'TemplateLiteral']);
+const stringAssignmentTypes = Object.freeze(['&&=', '||=', '??=']);
+
+export const assignmentOperatorMutator: NodeMutator = {
+  name: 'AssignmentOperator',
+
+  *mutate(path) {
+    if (path.isAssignmentExpression() && isSupportedAssignmentOperator(path.node.operator) && isSupported(path.node)) {
+      const mutatedOperator = AssignmentOperators[path.node.operator];
+      const replacement = types.cloneNode(path.node, false);
+      replacement.operator = mutatedOperator;
+      yield replacement;
+    }
+  },
+};
+
+function isSupportedAssignmentOperator(operator: string): operator is keyof typeof AssignmentOperators {
+  return Object.keys(AssignmentOperators).includes(operator);
+}
+
+function isSupported(node: types.AssignmentExpression): boolean {
+  // Excludes assignment operators that apply to strings.
+  if (stringTypes.includes(node.right.type) && !stringAssignmentTypes.includes(node.operator)) {
+    return false;
+  }
+
+  return true;
+}

packages/instrumenter/src/mutators/mutate.ts

@@ -13,6 +13,7 @@ import { unaryOperatorMutator } from './unary-operator-mutator';
 import { updateOperatorMutator } from './update-operator-mutator';
 import { regexMutator } from './regex-mutator';
 import { optionalChainingMutator } from './optional-chaining-mutator';
+import { assignmentOperatorMutator } from './assignment-operator-mutator';
 
 export const allMutators: NodeMutator[] = [
   arithmeticOperatorMutator,
@@ -29,4 +30,5 @@ export const allMutators: NodeMutator[] = [
   updateOperatorMutator,
   regexMutator,
   optionalChainingMutator,
+  assignmentOperatorMutator,
 ];

packages/instrumenter/test/unit/mutators/assignment-operator-mutator.spec.ts

@@ -0,0 +1,75 @@
+import { expect } from 'chai';
+
+import { assignmentOperatorMutator as sut } from '../../../src/mutators/assignment-operator-mutator';
+import { expectJSMutation } from '../../helpers/expect-mutation';
+
+describe(sut.name, () => {
+  it('should have name "AssignmentOperator"', () => {
+    expect(sut.name).eq('AssignmentOperator');
+  });
+
+  it('should mutate += and -=', () => {
+    expectJSMutation(sut, 'a += b', 'a -= b');
+    expectJSMutation(sut, 'a -= b', 'a += b');
+  });
+
+  it('should mutate *=, %= and /=', () => {
+    expectJSMutation(sut, 'a *= b', 'a /= b');
+    expectJSMutation(sut, 'a /= b', 'a *= b');
+    expectJSMutation(sut, 'a %= b', 'a *= b');
+  });
+
+  it('should mutate *=, %= and /=', () => {
+    expectJSMutation(sut, 'a *= b', 'a /= b');
+    expectJSMutation(sut, 'a /= b', 'a *= b');
+    expectJSMutation(sut, 'a %= b', 'a *= b');
+  });
+
+  it('should mutate <<=, >>=, &= and |=', () => {
+    expectJSMutation(sut, 'a *= b', 'a /= b');
+    expectJSMutation(sut, 'a /= b', 'a *= b');
+    expectJSMutation(sut, 'a %= b', 'a *= b');
+  });
+
+  it('should mutate &&=, ||= and ??=', () => {
+    expectJSMutation(sut, 'a &&= b', 'a ||= b');
+    expectJSMutation(sut, 'a ||= b', 'a &&= b');
+    expectJSMutation(sut, 'a ??= b', 'a &&= b');
+  });
+
+  it('should not mutate a string literal unless it is &&=, ||=, ??=', () => {
+    expectJSMutation(sut, 'a += "b"');
+    expectJSMutation(sut, 'a -= "b"');
+    expectJSMutation(sut, 'a *= "b"');
+    expectJSMutation(sut, 'a /= "b"');
+    expectJSMutation(sut, 'a %= "b"');
+    expectJSMutation(sut, 'a <<= "b"');
+    expectJSMutation(sut, 'a >>= "b"');
+    expectJSMutation(sut, 'a &= "b"');
+    expectJSMutation(sut, 'a |= "b"');
+  });
+
+  it('should mutate a string literal using &&=, ||=, ??=', () => {
+    expectJSMutation(sut, 'a &&= "b"', 'a ||= "b"');
+    expectJSMutation(sut, 'a ||= "b"', 'a &&= "b"');
+    expectJSMutation(sut, 'a ??= "b"', 'a &&= "b"');
+  });
+
+  it('should not mutate string template unless it is &&=, ||=, ??=', () => {
+    expectJSMutation(sut, 'a += `b`');
+    expectJSMutation(sut, 'a -= `b`');
+    expectJSMutation(sut, 'a *= `b`');
+    expectJSMutation(sut, 'a /= `b`');
+    expectJSMutation(sut, 'a %= `b`');
+    expectJSMutation(sut, 'a <<= `b`');
+    expectJSMutation(sut, 'a >>= `b`');
+    expectJSMutation(sut, 'a &= `b`');
+    expectJSMutation(sut, 'a |= `b`');
+  });
+
+  it('should mutate string template using &&=, ||=, ??=', () => {
+    expectJSMutation(sut, 'a &&= `b`', 'a ||= `b`');
+    expectJSMutation(sut, 'a ||= `b`', 'a &&= `b`');
+    expectJSMutation(sut, 'a ??= `b`', 'a &&= `b`');
+  });
+});

packages/instrumenter/testResources/instrumenter/disabled.js.out.snap

@@ -75,7 +75,7 @@ function factorial(num) {
 
     while (stryMutAct_9fa48(\\"13\\") ? i > num : stryMutAct_9fa48(\\"12\\") ? i < num : stryMutAct_9fa48(\\"11\\") ? false : (stryCov_9fa48(\\"11\\", \\"12\\", \\"13\\"), i <= num)) {
       // Stryker disable next-line UpdateOperator: Infinite loop
-      o *= i++;
+      stryMutAct_9fa48(\\"15\\") ? o /= i++ : (stryCov_9fa48(\\"15\\"), o *= i++);
     }
 
     return o;