Variable Scope

What is scope? Your explanation should include the idea of global vs. block scope.
  Scope refers to the area in which a variable is active or accessible. Block scope refers to variables that a defined within
  a block and thus only accessible within the block. Global variables are defined outside a block and are accessible throughout
  a file and even between files, provided the global variable is defined in a file that is read before another file that
  uses that variable.
  
Why are global variables avoided?
  The problem with global variables is that they can be changed by one function and that variable might then be used in
  another function and doesn't expect that the value of the variable might have changed. Bugs and unexpected side effects
  could occur.
  
Explain JavaScript's strict mode
  Strict mode forces you to use const or let when defining variables, thus avoiding the unintended creation of global
  variables.
  
What are side effects, and what is a pure function?
  Side effects are unintended consequences, like if you change the value of a variable but that variable is used some-
  place else and the value change alters the output of that second function. A pure function is one that has no unintended
  side effects and is also determinate, meaning it always generates the same output given the same input.