challenge: in your own words

scope

scope reffers to whether or not a variable is made, and called upon in a single function; or if the variable is made outside of a function 
and called upon in multiple functions. 

global variables are avoided because there is a chance that the varible will be modified from a source outside of the file it was created, 
and cause errors or bugs due to its' new modified value. 

javascripts strict mode will ensure that you always use let or const when declaring a new variable. 

a side effect is when a function reaches outside of its local scope and modifies the value of a variable that is in a global scope. 

a function is pure when it is both determinate and has no side effects.