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
br>‘use strict’;
x = 3.14; // will throw an error
Output:
// Using an object, without declaring it, is not allowed:
‘use script’;
x = {p1:10, p2:20}; // will throw an error
Output:
let x = 3.14;
// Deleting a function is also not allowed
‘use strict’;
function x(p1, p2) {};
delete x; // will throw an error
Output:
function x(p1, p1) {}; // will throw an error
Output:
let x = 010; // will throw an error
Output:
let x = 10; // will throw an error
Output:
let obj = {};
Object.defineProperty(obj, “x”, {value:0, writable:false});
obj.x = 3.14; //will throw an error
Output:
let obj = {get x() {return 0} };
obj.x = 3.14; // will throw an error
Output:
delete Object.prototype; // will throw an error
Output:
let eval = 3.14; // will throw an error
Output:
let arguments = 3.14; // will throw an error
Output:
with (Math){x = cos(2)}; // will throw an error
Output:
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