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JavaScript's **strict mode** is a way to _opt in_ to a restricted variant of JavaScript, thereby implicitly opting-out of "{{Glossary("Sloppy_mode", "sloppy mode")}}". Strict mode isn't just a subset: it _intentionally_ has different semantics from normal code.
Strict mode for an entire script is invoked by including the statement `"use strict";` before any other statements.