What does it means ?
Defines that JavaScript code should be executed in “strict mode”.
How to use it ?
Strict mode is declared by adding “use strict”; to the beginning of a script or a function.
Examples
- 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 declared. Eg for fix it var x = x = 3.14;
“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
}
Why use Strict Mode? Why it is a good practice ?
This will avoid a lot of errors as:
- 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.
What it not allowed in Strict Mode ?
- Using a variable or object without declaration;
x = 3.14;
OR
x = {p1:10, p2:20};
- Deleting a variable (or object), or function;
var x = 3.14;
delete x;
OR
function x(p1, p2) {};
delete x;
- Duplicating a parameter name;
function x(p1, p1) {};
- Octal numeric literals;
var x = 010;
-
Escape characters;
var x = \010;
-
Writing to a get-only or read-only property;
var obj = {};
Object.defineProperty(obj, “x”, {value:0, writable:false});
obj.x = 3.14;
OR
var obj = {get x() {return 0} };
obj.x = 3.14;
-
Deleting an undeletable property;
delete Object.prototype;
-
The string “eval” cannot be used as a variable
var eval = 3.14;
-
The string “arguments” cannot be used as a variable:
var arguments = 3.14;
-
The with statement
with (Math){x = cos(2)};
-
For security reasons, eval() to create variables in the scope from which it was called
eval (“var x = 2”);
alert (x);
