Added article on native prototype extension

Author: James Allardice

articles.md

@@ -309,7 +309,7 @@ the filename of the new article next to it.
  - '{a}' is only available in Mozilla JavaScript extensions (use moz option)
  - '{a}' is only available in ES6 (use esnext option)
  - `bad-declaration.md` - ~~You might be leaking a variable ({a}) here~~
- - Extending prototype of native object
+ - `extending-native.md` - ~~Extending prototype of native object~~
  - `invalid-typeof-value.md` - ~~Invalid typeof value~~
  - '{a}' is already defined in outer scope
  - A generator function shall contain a yield statement
@@ -365,7 +365,7 @@ the filename of the new article next to it.
  - Use '===' to compare with 'null'
  - `eval.md` - ~~eval can be harmful~~
  - `exception-assignment.md` - ~~Do not assign to the exception parameter~~
- - {{a}} prototype is read only, properties should not be added
+ - `extending-native.md` - ~~{{a}} prototype is read only, properties should not be added~~
  - Redundant double negation in {{a}}
  - Gratuitous parentheses around expression
  - `unnecessary-semicolon.md` - ~~Unnecessary semicolon~~

message-articles/extending-native.md

@@ -0,0 +1,91 @@
+<!---
+{
+    "titles": [
+        "Extending prototype of native object: '{a}'",
+        "{a} prototype is read only, properties should not be added",
+        "W121"
+    ],
+    "slugs": [
+        "extending-prototype-of-native-object",
+        "a-prototype-is-read-only",
+        "w121"
+    ],
+    "linters": [
+        "jshint",
+        "eslint"
+    ],
+    "author": "jallardice"
+}
+-->
+
+### When do I get this error?
+
+The "Extending prototype of native object: '{a}'" error, and the alternative
+"{a} prototype is read only, properties should not be added" error, is thrown
+when JSHint (only versions 2.3.0 and above) or ESLint encounters **a assignment
+to a property of the `prototype` of a native object*. JSHint will only raise
+this warning if the `freeze` option is set to `true`. The following example
+defines a `reverse` method on the native `String` prototype:
+
+<!---
+{
+    "linter": "jshint"
+}
+-->
+```javascript
+/*jshint freeze: true */
+String.prototype.reverse = function () {
+    "use strict";
+    return this.split("").reverse().join("");
+};
+```
+
+ESLint will also issue the warning when the `Object.defineProperty` method is
+used. JSHint does not warn in this situation:
+
+<!---
+{
+    "linter": "eslint"
+}
+-->
+```javascript
+/*jshint freeze: true */
+Object.defineProperty(String.prototype, "reverse", {
+    value: function () {
+        "use strict";
+        return this.split("").reverse().join("");
+    }
+});
+```
+
+*Side note*: the implementation of string reversal above is naive because it
+fails to take into account the way characters are encoded internally in
+JavaScript. See [this Stack Overflow answer][reverse] for a great explanation.
+
+### Why do I get this error?
+
+This error is raised to highlight the use of a technique commonly regarded as
+**bad practice**. By defining custom properties on native prototypes you can
+easily introduce problems in old browsers (in particular Internet Explorer 8 and
+below). The first pattern shown above will result in an enumerable property on
+`String.prototype`. If this is done to `Object.prototype` the new property will
+be produced by the `for...in` construct causing unexpected iterations of the
+loop.
+
+It's also very easy to accidentally shadow custom native prototype methods. For
+example, imagine you have defined a `count` method on `Object.prototype` which
+returns the number of properties an object has. If any object defines its own
+`count` property as part of the program logic the prototype method will be
+shadowed and inaccessible via the normal member operator. This makes it easy to
+introduce bugs and can also affect third party code.
+
+In ESLint the rule that generates this warning is named `no-extend-native`. You
+can disable it by setting it to `0`, or enable it by setting it to `1`.
+
+In JSHint 1.0.0 and above you have the ability to ignore any warning with a
+[special option syntax][jshintopts]. The identifier of this warning is **W121**.
+This means you can tell JSHint to not issue this warning with the `/*jshint
+-W121 */` directive. You can also set the `freeze` option to `false`.
+
+[reverse]: http://stackoverflow.com/questions/958908/how-do-you-reverse-a-string-in-place-in-javascript/16776621#16776621
+[jshintopts]: http://jshint.com/docs/#options