what is js framework?

Question

In many framework they are using 'use strict';
start with (function(){
---------
-------
})();

Answer 1

The function is supposed to keep everything out from the global namespace.

Strict mode has more strict "rules" on execution and it throws more exceptions.
Mistakes like declaring global variables, repeated literal object keys and parameters are handled as errors.

Answer 2

Take a deep look at ECMAScript, you'll understand