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.
Practice Excercise Practice now