Scope is where the a variable is available in code. A variable with global scope is available anywhere in the code, whereas a variable with local scope is only available in the block within which it is declared.

Global variables are avoided because they increase the chances of unwanted side effects in the code.

Strict mode is a mode that throws errors when a variable is declared without using the let or const keyword.

Side effects are changes of values outside of a function's local scope. A pure function is one that always returns the same value when given the same input and also has no side effects.