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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 "sloppy mode" Strict mode makes several changes to normal JavaScript semantics. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do not support it. The entire contents of JavaScript modules are automatically in strict mode, with no statement needed to initiate it. To invoke strict mode for an entire script, put the exact statement "use strict" before any other statements.