By marking the top of the script block or function with the "use strict” directive,
the compiler will provide early warnings and errors for common coding mistakes,
and restrict the usage of some dark corners of the JavaScript language. 

Let you see how the following sample code interact with Strict Mode. 

Code 

"use strict";
a = 100;
b = 200;
document.write(a + b);


Results
Error was detected; no results were produced.

Errors
Uncaught ReferenceError: a is not defined

Expected Code

"use strict";
var a = 100;
var b = 200;
document.write(a + b);

Results
300

Errors
No errors. 
(Note that Strict Mode is still under development in many browsers.)