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):
"use strict";
x = 3.14; // This will cause an error
because x is not declaredExample
"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):
Runnable example
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 errorObjects are variables too.
Using an object, without declaring it, is not allowed:
"use strict";
x = {p1:10, p2:20}; // This will cause an errorDeleting a variable (or object) is not allowed.
Runnable example
"use strict";
let x = 3.14;
delete x; // This
will cause an errorDeleting a function is not allowed.
Runnable example
"use strict";
function x(p1, p2) {};
delete x;
// This will cause an errorDuplicating a parameter name is not allowed:
Runnable example
"use strict";
function x(p1, p1) {}; // This will cause an errorRunnable example
"use strict";
let x = 010; // This
will cause an errorRunnable example
"use strict";
let x = "\010"; // This will cause an errorOctal escape characters are not allowed
Writing to a read-only property is not allowed:
Runnable example
"use strict";
const obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});
obj.x = 3.14; // This
will cause an errorWriting to a get-only property is not allowed:
Runnable example
"use strict";
const obj = {get x() {return 0} };
obj.x = 3.14; // This
will cause an errorDeleting an undeletable property is not allowed:
"use strict";
delete Object.prototype; // This will cause an errorThe word eval cannot be used as a variable:
Runnable example
"use strict";
let eval = 3.14; // This will cause an errorThe word arguments cannot be used as a variable:
Runnable example
"use strict";
let arguments = 3.14; // This will cause an errorThe with statement is not allowed
"use strict";
with (Math){x = cos(2)}; // This will cause an errorFor security reasons, eval() is not allowed to create variables in the scope from which it was called.
"use strict";
eval ("x = 2");
alert (x); // This
will cause an errorRunnable example
"use strict";
eval ("var x = 2");
alert (x); // This
will cause an errorRunnable example
eval ("let x = 2");
alert (x); // This
will cause an errorThe 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):
Runnable example
"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.