JavaScript Use Strict

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 "use strict";  Defines that JavaScript code should be executed in "strict mode".

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The "use strict" Directive

The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript version 5).

It is not a statement, but a literal expression, ignored by JavaScript 1, 2 and 3.

The purpose of "use strict" is to indicate that the code should be executed in "strict mode".

With strict mode, you cannot, for example, use undeclared variables.

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Declaring Strict Mode

Strict mode is declared by adding "use strict"; to the beginning of a JavaScript file, or a JavaScript function.

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

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

Global declaration:

"use strict";
function testStrict(){
    var x;
    x = 3.14; // This does not cause an error.
}
x = 3.14; // This causes an error.

Local declaration:

function testStrict(){
   "use strict";
    x = 3.14; // This causes an error.
}
x = 3.14; // This does not cause an error.

	Strict mode is supported in: Internet Explorer from version 10. Firefox from version 4. Chrome from version 13. Safari from version 5.1. Opera from version 12.

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Why Strict Mode?

Strict mode makes it easier to write "secure" JavaScript.

Strict mode changes previously accepted "bad syntax" into real errors.

As an 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.

In normal JavaScript, a developer will not receive any error feedback assigning values to non-writable properties.

In strict mode, any assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object, will throw an error.

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Not Allowed in Strict Mode

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

x = 3.14; // This causes an error (if x has not been declared).

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

var testStrict = 3.14;
delete testStrict; // This causes an error.

Defining a property more than once, is not allowed:

var testStrict = {p1:10, p2:15, p1:20}; // This causes an error.

Duplicating a parameter name, is not allowed:

function testStrict(param1, param1) {}; // This causes an error.

Octal numeric literals and escape characters are not allowed:

var testStrict = 010;  // This causes an error.
var testStrict = \010; // This causes an error.

Writing to a read-only property is not allowed:

var testObj = {};
Object.defineProperties(testObj,"x",{value:0,writable:false});
testObj.x = 3.14; // This causes an error.

Writing to a get-only property is not allowed:

var testObj = {get x() {return 0} };
testObj.x = 3.14; // This causes an error.

Deleting an undeletable property is not allowed:

delete Object.prototype; // This causes an error.

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

var eval = 3.14; // This causes an error.

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

var arguments = 3.14; // This causes an error.

The with statement is not allowed:

with (Math){x = cos(2)}; // This causes an error.

Future reserved keywords are not allowed. These are:

• implements
• interface
• package
• private
• protected
• public
• static
• yield