Added JS[LH]int forin option article

Author: James Allardice

option-articles/forin.md

@@ -0,0 +1,109 @@
+<!---
+{
+    "titles": [
+        "forin"
+    ],
+    "slugs": [
+        "option-forin"
+    ],
+    "linters": [
+        "jslint",
+        "jshint"
+    ],
+    "author": "jallardice",
+    "subject": "option"
+}
+-->
+
+### What does this option do?
+
+In JSLint the `forin` option is used to allow the usage of unfiltered for-in
+statements. Since the for-in statement will enumerate properties from the
+prototype chain and not just from the object in question, it can potentially
+cause problems if other code is modifying native prototypes without your
+knowledge. In the following example we use a for-in loop to iterate over the
+elements of an array. This is commonly called out as bad practice because
+enumerable methods added to `Array.prototype` will be produced:
+
+<!---
+{
+    "linter": "jslint"
+}
+-->
+```javascript
+/*jslint forin: true */
+/*global doSomething */
+
+var arr = [],
+    i;
+
+for (i in arr) {
+    doSomething(arr[i]);
+}
+```
+
+The JSHint `forin` option is used to **require** the filtering of such loops.
+Here's the same example again:
+
+<!---
+{
+    "linter": "jshint"
+}
+-->
+```javascript
+/*jshint forin: true */
+/*global doSomething */
+
+var arr = [],
+    i;
+
+for (i in arr) {
+    doSomething(arr[i]);
+}
+```
+
+A "filtered" for-in statement is one that ensures only properties that belong to
+the object in question are operated on. This is usually achieved by wrapping the
+body of the for-in in an `if` statement that makes sure each property is an
+"own" property of the object:
+
+<!---
+{
+    "linter": "jshint"
+}
+-->
+```javascript
+/*jshint forin: true */
+/*global doSomething */
+
+var arr = [],
+    i;
+
+for (i in arr) {
+    if (arr.hasOwnProperty(i)) {
+        doSomething(arr[i]);
+    }
+}
+```
+
+### When should I use this option?
+
+With JSLint, if the `forin` option is not set, you'll get a "[The body of a for
+in should be wrapped in an if statement to filter unwanted properties from the
+prototype][forin]" error any time an unfiltered for-in statement is used. In
+JSHint the opposite is true and you'll receive the same error message for each
+unfiltered for-in when the option *is* set.
+
+Note that in JSHint this is an *enforcing* option which means JSHint does not
+apply it by default. If you do not explicitly set this option to `true` JSHint
+will allow the use of bitwise operators anywhere in your code.
+
+#### Recommendation
+
+ - **JSLint** - Do not set this option (you will not be able to use unfiltered
+   for-in statements).
+
+ - **JSHint** - Set this option to `true` (you will not be able to use unfiltered
+   for-in statements).
+
+[forin]: /the-body-of-a-for-in-should-be-wrapped-in-an-if-statement