Eplanation of `var` in loops in javascript.

var.js

// Normal variable:
var x = 7;

// Used like:
console.log(x); // => 7

// Variable declared in a loop
for(var i = 0; i < 3; i++) {
  // Used like:
  console.log(i); // => 0,1,2
}

// Loop with pre-declared variable:
var looper;
for(looper = 0; looper < 3; looper++) {
  console.log(looper); // => 0,1,2
}

// for ... in loops are the same:
var thing = { x: 1, y: 2, z: 3};
for(var prop in thing) { // Be careful with this one!
  console.log(prop); // x, y, z
}