If you looked at some of the more recent JavaScript samples or had a look at the Windows 8 JavaScript applications, you’ve probably seen the ‘use strict’ statement before. But what is it and why use it?
By applying the ‘use strict’ statement inside your JavaScript, you are enabling Strict Mode. Strict Mode offers a set of enhancements to the JavaScript language and introduces better error-checking into your code. When you use strict mode, you cannot, for example, use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible.
Restrictions
It introduces the following restrictions to your code(source: MSDN):
| Language Element | Restriction |
| Variable | Using a variable without declaring it. |
| Read-only property | Writing to a read-only property. |
| Non-extensible property | Adding a property to an object whose extensible attribute is set to false. |
| delete | Deleting a variable, a function, or an argument. Deleting a property whose configurable attribute is set to false. |
| Duplicating a property | Defining a property more than once in an object literal. |
| Duplicating a parameter name | Using a parameter name more than once in a function. |
| Future reserved keywords | Using a future reserved keyword as a variable or function name. |
| Octals | Assigning an octal value to a numeric literal, or attempting to use an escape on an octal value. |
| this | The value of this is not converted to the global object when it is null or undefined. |
| eval as an identifier | The string "eval" cannot be used as an identifier (variable or function name, parameter name, and so on). |
| Function declared inside a statement or a block | You cannot declare a function inside a statement or a block. |
| Variable declared inside an eval function | If a variable is declared inside an eval function, it cannot be used outside that function. |
| Arguments as an identifier | The string "arguments" cannot be used as an identifier (variable or function name, parameter name, and so on). |
| arguments inside a function | You cannot change the values of members of the local arguments object. |
| arguments.callee | Not allowed. |
| with | Not allowed. |
Where can I use Strict Mode?
Strict Mode is part of the new EcmaScript 5 standard. For a list of browsers that already support this feature: http://caniuse.com/use-strict . But even if you have to target some browsers that not yet support this standard, I recommend start using Strict Mode today. You’ll not get compiler errors on these browsers but it has no negative on your application and it forces you to follow a set of recommended best practices.
If you are a Windows 8 Metro style apps HTML/JS developer, Strict Mode is automatically added to the project templates in Visual Studio 11 as WinJS supports Strict Mode.
How to enable Strict Mode?
That’s the easiest part, just add the "use strict" directive to your code (don’t forget to add the quotes).
Remark: However I recommend to not apply the Strict Mode to the global scope as this will impact not only your scripts but also 3th party JavaScript libraries you include.