Challenge: in your own words

gistfile1.txt

What is scope? Your explanation should include the idea of global vs. local scope.
Scope is the area in which a variable is available. 
Local scope is when the variable is defined and used within a specific function. Local scope is re-created
each time the function is called.
Global scope is when the variable is defined outside of a function.
The interpreter follows a scope chain to check the availability of a variable. It will first check local scope; 
then it will check parent scope;  then it will check the global scope. 

Why are global variables avoided?
Use of global variables can make a function indeterminate. This means that a when a function is given the same input
two different times, it outputs two different results instead of reliably mapping input to output. This makes it difficult
to reason and reuse the function.
Also, global variables make it difficult to debug errors since global variables can reach across multiple files.

Explain JavaScript's strict mode.
By putting the string 'strict mode' at the top of a file or a function, one can ensure that a variable cannot
be declared without using either the keyword 'let' or 'const'. This makes it impossible to accidentally create
a global variable. 

What are side effects, and what is a pure function?
A side effect is any result of a function other than the returned value of the function. Functions can intentionally
have side effects, such as a function that changes the records of a database. However, unintended side effects
can make it difficult to reason about the function.
A pure function is a value-returning function that always returns the same value when called with the same arguments; it can 
be replaced with the returned value of the function itself and everything can still run the same eway.