How do you declare strict mode

The strict mode is declared by adding "use strict"; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.

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

and if you declare inside a function, it has local scope

x = 3.14; // This will not cause an error.
myFunction();

function myFunction() {
"use strict";
y = 3.14; // This will cause an error
}