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 earlier versions of JavaScript.

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.

	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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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.

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The "use strict"; Syntax

The syntax, for declaring strict mode, was designed to be compatible with older versions of JavaScript.

Compiling a numeric literal (4 + 5;) or a string literal ("John Doe";) in a JavaScript program has no side effects. It simply compiles to a non existing variable and dies.

So "use strict;" only matters to new compilers that "understand" the meaning of it.

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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.defineProperty(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

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Other Differences

In function calls like f(), the this value was the global object. In strict mode, it is now undefined.

For security reasons, in strict mode code, eval does not create a new variable in the scope from which it was called.

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

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Watch Out!

	The "use strict" directive is only recognized at the beginning of a script or a function. If you add two JavaScript files into one file, you will lose the effect of the directive in the second file.