Strict mode in JavaScript

Strict Mode is 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.

Benifits 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 – used in global scope for the entire script and can be applied to individual functions. Strict mode doesn’t work with block statements enclosed in {} braces.





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!";

NOTE: 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: non-strict plus strict looks non-strict. 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 the entire script
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

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• 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
• Using strict mode, don’t allow to use a variable without declaring it
// Using a variable, without declaring it, is not allowed:
‘use strict’;
x = 3.14; // will throw an error

Output:


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

Output:


• Deleting a variable (or object) and a function is not allowed
‘use strict’;
let x = 3.14;

// Deleting a function is also not allowed
‘use strict’;
function x(p1, p2) {};
delete x; // will throw an error

Output:


• Duplicating a parameter name is not allowed
‘use strict’;
function x(p1, p1) {}; // will throw an error

Output:


• Octal numeric literals are not allowed
‘use strict’;
let x = 010; // will throw an error

Output:





• Escape characters are not allowed
‘use strict’;
let x = 10; // will throw an error

Output:


• Writing to a read-only property is not allowed
‘use strict’;
let obj = {};
Object.defineProperty(obj, “x”, {value:0, writable:false});

obj.x = 3.14; //will throw an error

Output:


• Writing to a get-only property is not allowed
‘use strict’;
let obj = {get x() {return 0} };

obj.x = 3.14; // will throw an error

Output:


• Deleting an undeletable property is not allowed
‘use strict’;
delete Object.prototype; // will throw an error

Output:


• The string “eval” cannot be used as a variable
‘use strict’;
let eval = 3.14; // will throw an error

Output:


• The string “arguments” cannot be used as a variable
‘use strict’;
let arguments = 3.14; // will throw an error

Output:


• The with statement is not allowed
‘use strict’;
with (Math){x = cos(2)}; // will throw an error

Output:


• In function calls like f(), the 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