A scope is basically a place where a variable is defined. A global scope is when a variable is defined outside a function and can be used in another function and a local scope is when a variable is defined inside of a function. Global variables are avoided because they can make unintended side effects.
Javascript’s strict mode is when a variable is defined using words other than let or const. Side effects are functions that reaches into a parent scope from a child scope and alters the values that live there. A pure function is dependant and doesn’t alter anything.