Global Scope

Take this example:

var a = 42; 
var b = 57;

function example() {
  var a = b = 10; 
}

console.log(A = " + a, "B = " + b"; -> A = 42 B = 57
example(); 
console.log(A = " + a, "B = " + b"; -> A = 42 B = 10

Why?

This function could be re-written as:

//How the javascript compiler sees it: 
function example_clear() {
  var a = 10;
  b = 10;
}

//When we don't have the 'var' keyword next to 'b', it is going to accessing the 'b' 
//variable in the scope above it, which is the global scope.

When you don't use 'var' you are attaching it to the global object.

If you're in a function then var will create a local variable, "no var" will look up the scope chain until it finds the variable or hits the global scope (at which point it will create it):