ECMAScript 5 (ES5) strict mode
Internet Explorer 10 and Windows Store apps introduce support for ECMAScript 5 (ES5) strict mode, as specified in the ECMAScript Language Specification, 5th edition.
Opting into strict mode
To use strict mode, you must opt in by including the following statement before scripts or functions.
For more information about ES5 strict mode from Ecma International, download the ECMAScript 5 specification and look at the Annex C section, called "The Strict Mode of ECMAScript."