JS chapter 16

To declare a variable use var something = or var something;
Variables in JavaScript are lexically scoped, if nested then it is available in all those scopes
a var inside a scope takes precedence over the var with same name outside of scope, also called blocked scope

a workaround of blocked scopes is to introduce a new scope in a then block. Also called an IIFE or immediately invoked function expression 
An IIFE enables you to attach private data to a function and therefore dont have to declare a global variable

A globally scoped variable is avaiable to entire program
two disadvantages of gloabal var are the side effects - they are less robust, behave less predictably, and are less reusable
and all JS on a webpage share the same global variables so naming can clash with other var.

to avoid this JS has module systems dont interface with global scope

A closure is a function plus the connection to the scope in which the function was created.