JavaScript Use Strict
The "use strict" Directive
The "use strict" directive was new in ECMAScript version 5.
It defines that JavaScript code should be executed in "strict mode".
It is not a statement. It is 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.
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
Try it Yourself »
Example
"use strict";
myFunction();
function myFunction() {
y = 3.14; // This will also cause an error
because y is not declared
}
Try it Yourself »
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
}
Try it Yourself »
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();
Watch Out!
The "use strict" directive is only recognized at the beginning of a script or a function.
