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"use strict"; defines that JavaScript code should be executed in "strict mode".
"use strict" 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".
You can use "strict mode" in all your programs. It helps you to write cleaner code, like preventing you from using undeclared variables.
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).
Declared inside a function, it has local or function scope (only the code inside the function is in strict mode)
"use strict";
x = 3.14; // Error: x is not declared
a = 3.14; // No error
myFunction();
function myFunction() {
"use strict";
b = 3.14; // Error: y is not declared
}
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.
Using a variable or object, without declaring it, is not allowed.
"use strict";
a = 3.14; // Error
b = {p1:10, p2:20}; // Error
Deleting a variable or object is not allowed.
"use strict";
let a = 3.14;
let b = {p1:10, p2:20};
delete a; // Error
delete b; // Error
Deleting a function is not allowed.
"use strict";
function myFun(p1, p2) {};
delete myFun; // Error
Duplicating a parameter name is not allowed.
"use strict";
function x(p1, p1) {}; // Error
Octal numeric literals and Octal escape characters are not allowed
"use strict";
let a = 010; // Error
let b = "\010"; // 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; // Error
Writing to a get-only property is not allowed.
"use strict";
const obj = {};
const obj = {get a() {return 0} };
obj.a = 3.14; // Error
Deleting an undeletable property is not allowed.
"use strict";
delete Object.prototype; // Error
The words eval, arguments cannot be used as a variable.
"use strict";
let eval = 3.14; // Error
let arguments = 3.14; // Error
The with statement is not allowed.
"use strict";
with (Math){x = cos(2)}; // Error
For security reasons, eval() is not allowed to create variables in the scope from which it was called.
"use strict";
eval ("x = 2");
alert (x); // Error
In strict mode, eval() can not declare a variable using the var or let keyword.
"use strict";
eval ("var a = 2");
eval ("let b = 2");
alert (a); // Error
alert (b); // 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); // alert "undefined"
}
myFunction();
Keywords reserved for future JavaScript versions can NOT be used as variable names in strict mode.
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