JavaScript "Use Strict" Directive

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

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.

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.

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

"use strict";
x = 3.14;       // This will cause an error because x is not declared

Example

"use strict";
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):

x = 3.14;       // This will not cause an error.
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.

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:

"use strict";
x = 3.14;                // This will cause an error

Objects are variables too.

Using an object, without declaring it, is not allowed:

"use strict";
x = {p1:10, p2:20};      // This will cause an error

Deleting a variable (or object) is not allowed.

"use strict";
let x = 3.14;
delete x;                // This will cause an error

Deleting a function is not allowed.

"use strict";
function x(p1, p2) {};
delete x;                // This will cause an error

Duplicating a parameter name is not allowed:

"use strict";
function x(p1, p1) {};   // This will cause an error

Octal numeric literals are not allowed:

"use strict";
let x = 010;             // This will cause an error

Octal escape characters are not allowed:

"use strict";
let x = "\010";            // This will cause an error

Writing to a read-only property is not allowed:

"use strict";
const 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:

"use strict";
const obj = {get x() {return 0} };

obj.x = 3.14;            // This will cause an error

Deleting an undeletable property is not allowed:

"use strict";
delete Object.prototype; // This will cause an error

The word eval cannot be used as a variable:

"use strict";
let eval = 3.14;         // This will cause an error

The word arguments cannot be used as a variable:

"use strict";
let arguments = 3.14;    // This will cause an error

The with statement is not allowed:

"use strict";
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.

In strict mode, a variable can not be used before it is declared:

"use strict";
eval ("x = 2");
alert (x);      // This will cause an error

In strict mode, eval() can not declare a variable using the var keyword:

"use strict";
eval ("var x = 2");
alert (x);    // This will cause an error

eval() can not declare a variable using the let keyword:

eval ("let 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):

"use strict";
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
"use strict";
let public = 1500;      // This will cause an error

Note:

The "use strict" directive is only recognized at the beginning of a script or a function.