Strict mode applies to entire scripts or to individual functions.
```
'use strict';
const v = "Hi! I'm a strict mode script!";
```
Strict mode makes several changes to normal JavaScript semantics:
* Eliminates some JavaScript silent errors by changing them to throw errors.
* Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.
* Prohibits some syntax likely to be defined in future versions of ECMAScript.