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The "use strict" Directive
"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.
"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.
"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 declared"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
}
The "use strict"; SyntaxThe 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 aJavaScript 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.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.Formula
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, 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 error
Deleting a variable (or object) is not allowed.
"use strict";
let x = 3.14;
delete x; // This will cause an errorDeleting 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 errorOctal numeric literals are not allowed:
"use strict";
let x = 010; // This will cause an errorOctal escape characters are not allowed:
"use strict";
let x = "\010"; // This will cause an errorFormula
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 errorFormula
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 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:
"use strict";
let eval = 3.14; // This will cause an errorThe word arguments cannot be used as a variable:
"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 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 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):
"use strict";
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.
implements interface let package private protected public static yield
"use strict";
let public = 1500; // This will cause an errorWatch Out! The "use strict" directive is only recognized at the beginning of a script or a function.