What is the ‘Strict’ mode in JavaScript and how can it be enabled? code example

Example: What is strict mode in Java Script ?

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.

Strict mode makes it easier to write "secure" JavaScript.