For entire scripts

Strict mode can be applied on entire scripts by placing the statement "use strict"; before any other statements.

"use strict";
// strict mode now applies for the rest of the script

Strict mode is only enabled in scripts where you define "use strict". You can combine scripts with and without strict mode, because the strict state is not shared among different scripts.

Note: All code written inside ES2015+ modules and classes are strict by default.