Scope

Scope is a set of rules that determine how widely accesible a variable is in a program. By accessible I mean that a variable's
value could be used or changed. 

Block scope determines that a variable is only accessible within a block. Global scope is when a variable is accesible through out a program.
Global can also sometimes be accessed in different files.

Global variables are avoided because they can cause code to become indeterminate (having different results under different conditions) 
which leads to errors and code that is hard to debug.

Strict mode requires that every declared variable in a program must use the keywords "let" or "const" in their declaration. 
By default, it's a best practice to put 'use strict' at the top of your program.

A side effect is when a function makes changes to a value outside of its scope. A function that always returns the same results with the same inputs is a determinate function. 
A pure function is determinate and has no side effects. You should generally try to use pure functions.