undefined is a variable that has been declared but no value exists and is a type of itself ‘undefined’.
null is a value of a variable and is a type of object.
We use ‘console.log();’ and ‘type of’ to check if a variable is undefined or null.
undeclared variables is a variable that has been declared without ‘var’ keyword.
testVar = ‘hello world’;
as opposed to
var testVar = ‘hello world’;
When former code is executed, undeclared variables are created as global variable and they are configurable (ex. can be deleted).