Introduction:

Strict Mode is one of the new feature introduced in ECMAScript5 edition.

Strict Mode is a group of the language that remove disfavor features. The Strict mode is not mandatory, it means that if we want our code to run in the strict mode, we can declare the expression as once per function or for the whole program by adding following string:

"use strict";

The Strict Mode (use strict) Declaration:

The main objective of use strict is to specify that the code or query should run in Strict Mode.
We can use the strict mode by adding use strict; at the starting of a function, a program, or a file. The main scope of a Strict Mode statements depends on the environment or events.

For Example -

If use strict; is mention in global environment or context, all the code in the program will execute in strict mode.

  "use  strict";

  function  testStrictFunction(){
       var  testVal = 3;
    return  testVal;
  }
  //  This causes a syntax error.
  testVal  = 5;

If use strict; is mention in a function, all the code in the function will execute in strict mode.

  function  testStrictFunction(){
    "use  strict";

    //This causes a syntax error.
    var  testVal = 3;
    return  testVal;
  }

  testVal  = 5;

The Strict Mode Limitation:

As we already mentioned that its a new feature of EcmaScript5, so there are few limitation rise in Script Mode implementation. These are given below.

Few Examples of Strict Mode:

=> Example - 01

(function() {
    "use strict";
    var a;
    var b;
    function bar() {
        x = 5; //Strict Mode will throw an error for implicit globals
    }
    bar(); //ReferenceError (Strict Mode is enforced)
})();

=> Example - 02

var container = function(e) {
    'use strict';
    var deleteNonConfigurable = function () {
        var obj = {};
        Object.defineProperty(obj, "name", {
            configurable: false
        });
            delete obj.name; //Will throw TypeError in Strict Mode
    }
    return deleteNonConfigurable;
}
wrapper()(); //TypeError (Strict Mode enforced)