Hoisting

Variable Hoisting:

• Hoisting means that when the Javascript runtime sees a 'var' keyword, it figures it out before it execute the function.
• Simplify by declaring 'var a', The var a is bound to the scope (function) before the function is executed

Example 1:

function example() {
  //'a' is an undeclared variable
  console.log(a); -> Crash as value is not found, walk up the scope...and cannot find it..
}

example();

Example 2:

function example() {
  a = 10; -> Referred as a left hand reference, I am trying to put something in memory. It is going to walk up the scope 
             chain, if it hits the global scope, it will attach it to the global scope. 
  console.log(a);  -> Right hand reference -> Will result in 10
}

example();

Example 3:

function example() {
  console.log(a);  -> Results in 'undefined' 
  var a; 
}
example();

Example 4:

function example() {
  console.log(a);  -> Results in 'undefined' 
  var a = 10; 
}
example();

Example 5:

Why was example 4 results in 'undefined?

var a = 10 actually means:

var a; -> Variable declaration (It will be undefined before it execute the function. 
a = 10; -> Variable assignment, it gets executed in order

//So for:
function example() {
  console.log(a); 
  var a = 10; 
}

//What the compiler sees is:
function example() {
  var a; -> Var a is not top 
  console.log(a);  -> Results in 'undefined' 
  a = 10; 
}