Scope is how your declared variable can be accessed at a differen point in your code.  Scope is categorized into two types: global scope or
local scope.  Global scope can be accessed every point in your code.  Therefore, global variables are declared outside a function in JavaScript.
Local scope is a variable defined in the function.  The local variable is access inside the function.  Of the two types of variable, global
variables are best avoided since they have strong tendency for side effects.  Side effect is when a function reaches outside of its local scope
and into parent scope and changes the values of the variables that are in these scopes. Pure function is when your code is free of side effects
and is determinate.  Beside globa or local scope, there is strict mode.  To declare strict mode, 'use strict' is declare at the top
of the code.  Also, undeclared variables are not allowed in strict mode since they cause errors.