Scope is the area in which the program executes or is affected by. In terms of global vs local, global is for the whole program throughout it all
Where as local or block scope is only for the function you write in the program.
Global variables are avioded because you want to avoid side effects which is when a value is changed from the local scope to the parent one or indeterminantes where a variable will return a certain value sometimes and another value other times
when one enables strict mode throught the whole program if you dont change the variable with let or const an error will occur
side effects are when a set value in a local scope also changes in the parent scope
to be pure you need to have no side effects and no indeterminants