Added 'function form of use strict' article

Author: James Allardice

articles/function-strict.md

@@ -0,0 +1,96 @@
+<!---
+{
+    "titles": [
+        "Use the function form of 'use strict'",
+        "W097"
+    ],
+    "slugs": [
+        "use-the-function-form-of-use-strict",
+        "w097"
+    ],
+    "linters": [
+        "jslint",
+        "jshint",
+        "eslint"
+    ],
+    "author": "jallardice"
+}
+-->
+
+### When do I get this error?
+
+The "Use the function form of 'use strict'" error is thrown when JSLint, JSHint
+or ESLint encounters a **strict mode directive in the outermost scope of the
+code**. In the following example we use a strict mode directive in the global
+scope to ensure the entire program runs in strict mode:
+
+<!---
+{
+    "linter": "jslint"
+}
+-->
+```javascript
+"use strict";
+function example() {
+    return true;
+}
+```
+
+### Why do I get this error?
+
+This error is raised to highlight a **potentially dangerous piece of code**.
+It's common and good practice to concatenate multiple JavaScript files into one
+to reduce HTTP requests in production environments. If one of those files has a
+global strict mode directive and others do not, the concatenation would result
+in all scripts running in strict mode. If one the those scripts depends upon
+features that are disallowed in strict mode you may run into errors. Consider
+the following example which shows the previous script concatentated with another
+that relies upon features that are illegal in strict mode:
+
+<!---
+{
+    "linter": "jslint"
+}
+-->
+```javascript
+"use strict";
+function example() {
+    return true;
+}
+function another(a) {
+    return 010; // Octal literal, illegal in strict mode
+}
+```
+
+This example will cause a syntax error since octal literals are not allowed in
+strict mode. If we want to use strict mode in our script and still be able to
+concatenate it with others we need to ensure our strict mode directive is not in
+the global scope. Since JavaScript only has function scope this means we need to
+place the directive within a function:
+
+<!---
+{
+    "linter": "jslint"
+}
+-->
+```javascript
+function example() {
+    "use strict";
+    return true;
+}
+```
+
+This has the added benefit of allowing you to control exactly which parts of
+your own script run in strict mode. However, a common technique is to wrap your
+entire program in an immediately invoked function expression to constrain it to
+its own function scope and help prevent pollution of the global scope.
+
+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 **W069**.
+This means you can tell JSHint to not issue this warning with the `/*jshint
+-W097 */` directive. You can also set the `sub` option to `true`.
+
+In ESLint the rule that generates this warning is named `no-global-strict`. You
+can disable it by setting it to `0`, or enable it by setting it to `1`.
+
+[jshintopts]: http://jshint.com/docs/#options