- JavaScript Introduction
- JavaScript Where To
- JavaScript Output
- JavaScript Statements
- JavaScript Syntax
- JavaScript Comments
- JavaScript Variables
- JavaScript Let
- JavaScript Const
- JavaScript Operators
- JavaScript Assignment
- JavaScript Data Types
- JavaScript Functions
- JavaScript Objects
- JavaScript Events
- JavaScript Strings
- JavaScript String Methods
- JavaScript Numbers
- JavaScript Number Methods
- JavaScript Arrays
- JavaScript Array Const
- JavaScript Array Methods
- JavaScript Sorting Arrays
- JavaScript Array Iteration
- JavaScript Date Objects
- JavaScript Date Formats
- JavaScript Get Date Methods
- JavaScript Set Date Methods
- JavaScript Math Object
- JavaScript Random
- JavaScript Booleans
- JavaScript Comparison And Logical Operators
- JavaScript If Else And Else If
- JavaScript Switch Statement
- JavaScript For Loop
- JavaScript Break And Continue
- JavaScript Type Conversion
- JavaScript Bitwise Operations
- JavaScript Regular Expressions
- JavaScript Errors
- JavaScript Scope
- JavaScript Hoisting
- JavaScript Use Strict
- The JavaScript This Keyword
- JavaScript Arrow Function
- JavaScript Classes
- JavaScript JSON
- JavaScript Debugging
- JavaScript Style Guide
- JavaScript Common Mistakes
- JavaScript Performance
- JavaScript Reserved Words
- JavaScript Versions
- JavaScript History
- JavaScript Forms
- JavaScript Validation API
- JavaScript Objects
- JavaScript Object Properties
- JavaScript Function Definitions
- JavaScript Function Parameters
- JavaScript Function Invocation
- JavaScript Closures
- JavaScript Classes
- Java Script Async
- JavaScript HTML DOM
- The Browser Object Model
- JS Ajax
- JavaScript JSON
- JavaScript Web APIs
- JS Vs JQuery
JavaScript Use Strict
The "use Strict" Directive
The "use strict" directive was new in 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 can not, for example, use undeclared variables.
All modern browsers support "use strict" except Internet Explorer 9 and lower:
The numbers in the table specify the first browser version that fully supports the directive.
You can use strict mode in all your programs. It helps you to write cleaner code, like preventing you from using undeclared variables.
"use strict" is just a string, so IE 9 will not throw an error even if it does not understand it.
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Declaring Strict Mode
Strict mode is declared by adding "use strict"; to the beginning of a script or a function.
Declared at the beginning of a script, it has global scope (all code in the script will execute in strict mode):
Example
x = 3.14; // This will cause an error because x is not declared
Example
myFunction();
function myFunction() {
y = 3.14; // This will also cause an error because y is not declared
}
Declared inside a function, it has local scope (only the code inside the function is in strict mode):
myFunction();
function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}
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.
Not Allowed in Strict Mode
Using a variable, without declaring it, is not allowed:
x = 3.14; // This will cause an error
Objects are variables too.
Using an object, without declaring it, is not allowed:
x = {p1:10, p2:20}; // This will cause an error
Deleting a variable (or object) is not allowed.
var x = 3.14;
delete x; // This will cause an error
Deleting a function is not allowed.
function x(p1, p2) {};
delete x; // This will cause an error
Duplicating a parameter name is not allowed:
function x(p1, p1) {}; // This will cause an error
Octal numeric literals are not allowed:
var x = 010; // This will cause an error
Octal escape characters are not allowed:
var x = "\010"; // This will cause an error
Writing to a read-only property is not allowed:
var obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});
obj.x = 3.14; // This will cause an error
Writing to a get-only property is not allowed:
var obj = {get x() {return 0} };
obj.x = 3.14; // This will cause an error
Deleting an undeletable property is not allowed:
delete Object.prototype; // This will cause an error
The word eval cannot be used as a variable:
var eval = 3.14; // This will cause an error
The word arguments cannot be used as a variable:
var arguments = 3.14; // This will cause an error
The with statement is not allowed:
with (Math){x = cos(2)}; // This will cause an error
For security reasons, eval() is not allowed to create variables in the scope from which it was called:
eval ("var x = 2");
alert (x); // This will cause an error
The this keyword in functions behaves differently in strict mode.
The this keyword refers to the object that called the function.
If the object is not specified, functions in strict mode will return undefined and functions in normal mode will return the global object (window):
function myFunction() {
alert(this); // will alert "undefined"
}
myFunction();
Future Proof!
Keywords reserved for future JavaScript versions can NOT be used as variable names in strict mode.
These are:
- implements
- interface
- let
- package
- private
- protected
- public
- static
- yield
var public = 1500; // This will cause an error
Watch Out!
The "use strict" directive is only recognized at the beginning of a script or a function.
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