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Strict mode in JavaScript

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  • Difficulty Level : Easy
  • Last Updated : 20 Nov, 2021
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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. 

// Whole-script strict mode syntax
'use strict';
 let v = "strict mode script!";

This syntax has a flow, it isn’t possible to blindly concatenate non-conflicting scripts. Consider concatenating a strict mode script with a non-strict mode script. The entire concatenation looks strict, the inverse is also true. The non-strict plus strict looks non-strict. The concatenation of strict mode scripts with each other is fine, and concatenation of non-strict mode scripts is fine. Only concatenating strict and non-strict scripts is problematic. It is thus recommended that you enable strict mode on a function-by-function basis (at least during the transition period).

Using Strict mode for a function: Likewise, to invoke strict mode for a function, put the exact statement “use strict”; (or ‘use strict’;) in the function’s body before any other statements.

function strict() {

  // Function-level strict mode syntax
  'use strict';

  function nested() { return 'Javascript on GeeksforGeeks'; }

  return "strict mode function!  " + nested();
}
function notStrict() { return "non strict function"; }

Examples of using Strict mode: 

  • Example: In normal JavaScript, mistyping a variable name creates a new global variable. In strict mode, this will throw an error, making it impossible to accidentally create a global variable. 

Javascript




// Using a variable, without declaring it, is not allowed:
'use strict';
 x = 3.14;  // will throw an error


  • Output: 

  • Example: Using strict mode, don’t allow to use a variable without declaring it. 

Javascript




// Objects are variables too.
// Using an object, without declaring it, is not allowed:
'use strict';
 
  // Will throw an error
  x = {p1:10, p2:20};


  • Output: 

  • Example: Deleting a variable (or object) and a function is not allowed. 

Javascript




'use strict';
 let x = 3.14;
 
// Deleting a function is also not allowed
'use strict';
 function x(p1, p2) {};
 
 // Will throw an error
 delete x;     


  • Output: 

  • Example: Duplicating a parameter name is not allowed. 

Javascript




'use strict';
 
 // Will throw an error
 function x(p1, p1) {};  


  • Output: 

  • Example: Octal numeric literals are not allowed. 

Javascript




'use strict';
 
 // Will throw an error
 let x = 010;      


  • Output: 

  • Example: Escape characters are not allowed. 

Javascript




'use strict';
 
 // Will throw an error
 let x = \010;    


  • Output: 

  • Example: Writing to a read-only property is not allowed. 

Javascript




'use strict';
 let obj = {};
 Object.defineProperty(obj, "x", {value:0, writable:false});
 
 // Will throw an error
 obj.x = 3.14;   


  • Output: 

  • Example: Writing to a get-only property is not allowed. 

Javascript




'use strict';
 let obj = {get x() {return 0} };
 
 // Will throw an error
 obj.x = 3.14;    


  • Output: 

  • Example: Deleting an undeletable property is not allowed. 

Javascript




'use strict';
 
 // Will throw an error
 delete Object.prototype;


  • Output: 

  • Example: The string “eval” cannot be used as a variable. 

Javascript




'use strict';
 
 // Will throw an error
 let eval = 3.14; 


  • Output: 

  • Example: The string “arguments” cannot be used as a variable. 

Javascript




'use strict';
 
 // Will throw an error
 let arguments = 3.14;   


  • Output: 

  • Example: The with statement is not allowed. 

Javascript




'use strict';
 
 // Will throw an error
 with (Math){x = cos(2)};


  • Output: 

Note: In function calls like f(), this value was the global object. In strict mode, it is now undefined. In normal JavaScript, a developer will not receive any error feedback assigning values to non-writable properties.


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