Strict mode - JavaScript | MDN

Note: Sometimes you'll see the default, non-strict mode referred to as sloppy mode. This isn't an official term, but be aware of it, just in case.

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 isn't just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

Strict mode applies to entire scripts or to individual functions. It doesn't apply to block statements enclosed in {} braces; attempting to apply it to such contexts does nothing. eval code, Function code, event handler attributes, strings passed to setTimeout(), and related functions are either function bodies or entire scripts, and invoking strict mode in them works as expected.

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.

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