What is scope? Your explanation should include the idea of global vs. local scope.
  Scope is the scale of what a particular variable can interact with. For instance if a variable is delcared outside of a function the scope would be global. Any future functions that call that particular variable outside the function. Local scope is a variable declared only in inside the function.
Why are global variables avoided?
  Global variables can cause indeterminate functions, which mean that given the same inputs a function can produce multiple outputs.
Explain JavaScript's strict mode
  Strict mode assumes all variables decleared without let or const is an error.
What are side effects, and what is a pure function?
  Side effects are the unintented mutations of varibles. If a global variable is decleared and a function mutates the variable it could cause pure functions to become indeterminate.
Example below;

// tip calculator
const prices = 10;
const customers = 3;
// to make this simple set price and customers
let tip = 0.2;
// 20% tip as a baseline
let tax  = 0.0175;

let bill = prices * customers + tax;

function getTip() {
    bill += tip * bill;
    return bill;
}
function receiptPrint() {
    console.log(' Your Bill is = $'+ bill + '    Tip _____' + '   Total ______');
}

getTip();

receiptPrint();


/* the receipt would print the base bill with the tip included since the getTip function variable is global in scope. */