Use strict mode
The “use strict” directive changes code into the mode that adheres to the up-to-date standard. It looks like the line “use strict” or ‘use strict’ and is placed at the beginning of the script:
Besides, the directive is used in functions. In such cases, a strict mode can be executed in function scope. The action “use strict” can’t be possibly canceled. It’s not supported by versions IE9 and older.