Strict Mode was introduced in ECMAScript 5.0 and is intended to make programs simpler and free of errors. In strict mode, a more rigid set of syntax checks is used to either catch an undeclared variable or any other errors in your code. Strict mode changes semantics!
What is ECMAScript?
What if my browser, especially older versions, does not support Strict Mode?
StrictMode is declared as a string. For eg:
Globally as "use strict";
and in a function as
// code from DevCurry.com
Since the Strict mode is declared as a string, it is backward compatible. Older browsers simply ignore it, without raising any errors. However you should make sure to test your code in browsers that do and do not support strict mode.
Which browsers support Strict Mode?
As of this writing IE 10 (Preview 1), Firefox 4 & 5, Chrome 12, Webkit, Besen and Jurassic support Strict Mode. It’s here to stay, so get used to it!