Question

How to secure my JavaScript using "Strict mode"?

Answer 1

The "use strict" Directive

The "use strict" directive was new in ECMAScript version 5.

It 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".

With strict mode, you can not, for example, use undeclared variables.

All modern browsers support "use strict" except Internet Explorer 9 and lower

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):

Example

"use strict";

x = 3.14; // This will cause an error because x is not declared

Example

"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

}