Scope, simply put, determines the accessibility of variables. The two types of scope described in the reading are global and block. Block scope is used within a function and only can be accessed within that function. Global scope can be accessed from all scripts.
The use of global variables is avoided to prevent unintended side effects within a program. Declaring variables on a global level can lead to tricky and hard to track down bugs.
Strict mode is a tool within JavaScript that keeps the coder from making mistakes in relation to creating variables. Unless the coder uses a 'let' or 'const' statement to declare a variable the console will return an error.
A pure function is one that is both determinate and has no side effects. Side effects can occur when global variables are set and interact with other functions unintentionally.