JavaScript is everywhere. It’s run in web sites, as utilities on many operating systems, and in any host, like IE, Firefox, or Chrome and it’s a first class citizen in Windows 8 Metro style app development. Since JavaScript has been around for a long time, it still has a lot of baggage from when it was immature. Because of this, Strict Mode has become a feature in ECMAScript 5, or ES5 (JavaScript).
What is Strict Mode and why use it?
Because Strict Mode is ECMAScript, it is a standard, and a best practice, and highly recommended that you use it. In the future, Strict Mode will be your only choice, which is certainly for the best. Strict Mode offers up these enhancements to the JavaScript language:
- JavaScript throws errors if you try to use deprecated language elements
- Variable declaration is mandatory
- Disallows duplicate property and parameter names
- The eval() method is safer to use, but still considered evil in some cases.
- Deprecates the with statement
- JavaScript throws errors if you try to assign a value to a read only property
- Decreases the global namespace pollution
Most ES5 compliant JavaScript runtimes support Strict Mode, and you can get full details at this in-depth article on Strict Mode at MSDN.
Where can I use Strict Mode?
You can use strict mode in any JavaScript host or engine that supports ES5 standards, and specifically, Strict Mode. Check out this comprehensive ES5 Compat Table on Github to see if your browser supports strict. You will see a lot of green lights, so there is wide support across browsers. This means if you are a web developer you can start using Strict Mode today.
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.
Enabling Strict Mode
Applying Strict Mode is very easy to do. Just add the “use strict” directive to your code (it’s a string literal so include the quotes). The physical location of the strict directive determines the scope of Strict Mode. Below is a table showing the relationship between Strict Mode and scope. This means you can turn Strict Mode on and off granularly in code.
| Code location | Scope |
| Beginning of any .js file | Global |
| Beginning of the script inside a <script> element. | <script> element |
| Preceding function block First line in function block |
Function |
As you can see, Strict Mode is flexible but try to keep Strict Mode out of a global setting where possible when dealing with Web pages (HTML, ASP.NET, PHP, etc…). If other scripts include your script using Strict Mode, then Strict Mode could apply to the included scripts and that can create problems. However, you can get the same effect as global Strict mode by wrapping your code like so:
“use strict”;
function strictFoo() {
alert(“I am in strict mode”);
}
function strictFoo2() {
alert(“Me too!”);
}
});
This creates a block around both functions and Strict Mode appears global to to the all code that exists in that block, essentially making strict container for that code.
Summary
Strict Mode is great because it catches common programming ID10T errors. These are the kind of errors everyone makes, regardless of experience, and often because of experience. It’s very easy, especially when moving from a compiled language, to miss some of the finer nuances of JavaScript. Strict Mode helps smooth out those JS language kinks.
Rachel Appel 