JAVASCRIPT

javascript use strict

// Whole-Script Strict Mode Syntax
'use strict';
var v = "Hi! I'm a strict mode script!";

what is "use strict"

The statement "use strict";  instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.

use strict

The statement "use strict";  instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.

List of features (non-exhaustive)
-Disallows global variables. (Catches missing var declarations and typos in variable names)

-Silent failing assignments will throw error in strict mode (assigning NaN = 5;)

-Attempts to delete undeletable properties will throw (delete Object.prototype)

-Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"})

-Function parameter names must be unique (function sum (x, x) {...})

-Forbids octal syntax (var x = 023; some devs assume wrongly that a preceding zero does nothing to change the number.)

-Forbids the with keyword

-eval in strict mode does not introduce new variables

-Forbids deleting plain names (delete x;)

-Forbids binding or assignment of the names eval and arguments in any form

-Strict mode does not alias properties of the arguments object with the formal parameters. (e.g. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. ) (See examples section below to understand the difference)

-arguments.callee is not supported

JavaScript Use Strict

"use strict";
myFunction();

function myFunction() {
  y = 3.14;   // This will also cause an error because y is not declared
}

use strict javascript

// File: myscript.js

'use strict';
var a = 2;
....

use strict javascript

function doSomething() {
    'use strict';
    ...
}

JavaScript "use strict"

'use strict';

// Error
myVariable = 9;

JavaScript "use strict"

myVariable = 9;