What is the Use of "Use Strict" in JavaScript?

"use strict";

It is an literal expression. The purpose of "use strict" is to indicate that the JavaScript code should be executed in "strict mode".

With strict mode, you cannot use undeclared variables.

i.e. Compiling a numeric literal (2 + 5;) or a string literal ("Arun Kumar";) in a JavaScript program has no side effects. It simply compiles to a non existing variable and dies.

Declaration and Scope of "use strict"

It is declared by adding "use strict" at the beginning of a JavaScript file or a JavaScript function. Its effects are similar to global and local variables in javascript.

Declared at the beginning of a JavaScript file, it has global scope means all code will execute in strict mode.

Declared inside a function, it has local scope i.e. only the code inside the function is in strict mode.

Global declaration:

 "use strict";
 x = 3.14;       // This will cause an error
 myFunction();   // This will also cause an error

 function myFunction() {
  x = 3.14;
 }

Local declaration:

 x = 3.14;       // This will not cause an error. 
 myFunction();   // This will cause an error

 function myFunction() {
    "use strict";
  x = 3.14;
 }
 

Expressions that are Not Allowed in Strict Mode

1) Using a variable (property or object) without declaring it, is not allowed:

 "use strict";
 x = 3.14;   // This will cause an error 
     // (if x has not been declared)

2) Deleting a variable, a function, or an argument, is not allowed.

 "use strict";
 x = 3.14;
 delete x;      // This will cause an error

3) Defining a property more than once, is not allowed:

 "use strict";
 var x = {p1:10, p1:20};   // This will cause an error
 

4) Duplicating a parameter name is not allowed:

 "use strict";
 function x(p1, p1) {};    // This will cause an error
 

5) Octal numeric literals and escape characters are not allowed:

 "use strict";
 var x = 010;             // This will cause an error
 var y = \010;            // This will cause an error
 

6) Writing to a read-only property is not allowed:

 "use strict";
 var obj = {};
 obj.defineProperty(obj, "x", {value:0, writable:false});
 
 obj.x = 3.14;            // This will cause an error
 

7) Writing to a get-only property is not allowed:

 "use strict";
 var obj = {get x() {return 0} };
 
 obj.x = 3.14;            // This will cause an error
 

8) Deleting an undeletable property is not allowed:

 "use strict";
 delete Object.prototype; // This will cause an error
 

9) The string "eval" cannot be used as a variable:

 "use strict";
 var eval = 3.14;         // This will cause an error
 

10) The string "arguments" cannot be used as a variable:

 "use strict";
 var arguments = 3.14;    // This will cause an error
 

11) The with statement is not allowed:

 "use strict";
 with (Math){x = cos(2)}; // This will cause an error
 

12) For security reasons, eval() are not allowed to create variables in the scope from which it was called:

 "use strict";
 eval ("var x = 2");
 alert (x)                // This will cause an error
 

Conclusion :

I hope that this article would have helped you in understanding the use of Use Strict in JavaScript. Your feedback and constructive contributions are always welcome.