Merge pull request #627 from amplii/no-param-reassign-with-props

[eslint config] [breaking] update best-practices config to prevent parameter object manipulation

README.md

@@ -629,6 +629,39 @@ Other Style Guides
   const y = function a() {};
   ```
 
+- [7.12](#7.12) <a name="7.12"></a> Never mutate parameters.
+
+  > Why? Overwriting parameters can lead to unexpected behavior, especially when accessing the `arguments` object. Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.
+
+  eslint rules: [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign.html).
+
+  ```javascript
+  // bad
+  function f1(a) {
+    a = 1;
+  }
+
+  function f2(a) {
+    if (!a) { a = 1; }
+  }
+
+  function f3(obj) {
+    obj.key = 1;
+  };
+
+  // good
+  function f4(a) {
+    const b = a || 1;
+  }
+
+  function f5(a = 1) {
+  }
+
+  function f6(obj) {
+    const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
+  };
+  ```
+
 **[⬆ back to top](#table-of-contents)**
 
 ## Arrow Functions

packages/eslint-config-airbnb/rules/best-practices.js

@@ -74,7 +74,9 @@ module.exports = {
     // var foo = 'Copyright \251';
     'no-octal-escape': 2,
     // disallow reassignment of function parameters
-    'no-param-reassign': 2,
+    // disallow parameter object manipulation
+    // rule: http://eslint.org/docs/rules/no-param-reassign.html
+    'no-param-reassign': [2, { 'props': true }],
     // disallow use of process.env
     'no-process-env': 0,
     // disallow usage of __proto__ property