Strict Mode was a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This strict context prevents certain actions from being taken and throws more exceptions. The statement “use strict”; instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.

Benefits of using ‘use strict’: Strict mode makes several changes to normal JavaScript semantics. 

• Strict mode eliminates some JavaScript silent errors by changing them to throw errors.
• Strict mode 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.
• Strict mode prohibits some syntax likely to be defined in future versions of ECMAScript.
• It prevents, or throws errors, when relatively “unsafe” actions are taken (such as gaining access to the global object).
• It disables features that are confusing or poorly thought out.
• Strict mode makes it easier to write “secure” JavaScript.

How to use strict mode: Strict mode can be used in two ways, remember strict mode doesn’t work with block statements enclosed in {} braces. 

• Used in global scope for the entire script.
• It can be applied to individual functions.

Using Strict mode for the entire script: To invoke strict mode for an entire script, put the exact statement “use strict”; (or ‘use strict’;) before any other statements.