Fix: don't modify operator precedence in operator-assignment autofixer (

#8358)

* Fix: don't modify operator precedence in operator-assignment autofixer

Previously, the operator-assignment autofixer could sometimes modify semantics or produce a syntax error due to different operator precedence. This commit updates the fixer to surround the right side of an assignment with parentheses if it has lower precedence than its new neighbor.

* Add additional test

lib/rules/operator-assignment.js

@@ -4,6 +4,12 @@
  */
 "use strict";
 
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const astUtils = require("../ast-utils");
+
 //------------------------------------------------------------------------------
 // Helpers
 //------------------------------------------------------------------------------
@@ -171,9 +177,20 @@ module.exports = {
                         if (canBeFixed(node.left)) {
                             const operatorToken = getOperatorToken(node);
                             const leftText = sourceCode.getText().slice(node.range[0], operatorToken.range[0]);
-                            const rightText = sourceCode.getText().slice(operatorToken.range[1], node.range[1]);
+                            const newOperator = node.operator.slice(0, -1);
+                            let rightText;
+
+                            // If this change would modify precedence (e.g. `foo *= bar + 1` => `foo = foo * (bar + 1)`), parenthesize the right side.
+                            if (
+                                astUtils.getPrecedence(node.right) <= astUtils.getPrecedence({ type: "BinaryExpression", operator: newOperator }) &&
+                                !astUtils.isParenthesised(sourceCode, node.right)
+                            ) {
+                                rightText = `${sourceCode.text.slice(operatorToken.range[1], node.right.range[0])}(${sourceCode.getText(node.right)})`;
+                            } else {
+                                rightText = sourceCode.text.slice(operatorToken.range[1], node.range[1]);
+                            }
 
-                            return fixer.replaceText(node, `${leftText}= ${leftText}${node.operator.slice(0, -1)}${rightText}`);
+                            return fixer.replaceText(node, `${leftText}= ${leftText}${newOperator}${rightText}`);
                         }
                         return null;
                     }

tests/lib/rules/operator-assignment.js

@@ -81,7 +81,8 @@ ruleTester.run("operator-assignment", rule, {
         "x = x === y",
         "x = x !== y",
         "x = x && y",
-        "x = x || y"
+        "x = x || y",
+        "x = x * y + z"
     ],
 
     invalid: [{
@@ -214,6 +215,31 @@ ruleTester.run("operator-assignment", rule, {
         output: "foo = foo ** bar",
         options: ["never"],
         errors: UNEXPECTED_OPERATOR_ASSIGNMENT
+    }, {
+        code: "foo *= bar + 1",
+        output: "foo = foo * (bar + 1)",
+        options: ["never"],
+        errors: UNEXPECTED_OPERATOR_ASSIGNMENT
+    }, {
+        code: "foo -= bar - baz",
+        output: "foo = foo - (bar - baz)",
+        options: ["never"],
+        errors: UNEXPECTED_OPERATOR_ASSIGNMENT
+    }, {
+        code: "foo += bar + baz",
+        output: "foo = foo + (bar + baz)", // addition is not associative in JS, e.g. (1 + 2) + '3' !== 1 + (2 + '3')
+        options: ["never"],
+        errors: UNEXPECTED_OPERATOR_ASSIGNMENT
+    }, {
+        code: "foo += bar = 1",
+        output: "foo = foo + (bar = 1)",
+        options: ["never"],
+        errors: UNEXPECTED_OPERATOR_ASSIGNMENT
+    }, {
+        code: "foo *= (bar + 1)",
+        output: "foo = foo * (bar + 1)",
+        options: ["never"],
+        errors: UNEXPECTED_OPERATOR_ASSIGNMENT
     }]
 
 });