Answer 1

strict mode enables strict features in the v8 engine. Short example of some features:

You can enable it globally by writing:

'use strict'; // strict mode enabled!

Per function you just include in function:

let myfunc = () => {
'use strict'; // strict mode enabled

b = 0; // broke
}

* You MUST declare a variable before using it (sane imo):

var x;
x = '0'; // ok
y = '';. // not ok

* es6 features are enabled (this is browser dependent), for node v4+ this is important.

* Performance, in some cases, is better.

Tere are more features as well, I'd check stack overflow for more!