what is strict mode in javascript?

The normal JavaScript is sometimes referred to as sloppy mode.

JavaScript's strict mode 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.

Invoking strict mode

 Strict mode applies to entire scripts or to individual functions. 

Strict mode for script 

Strict mode for functions 

Strict mode for modules 

Strict mode for classes