JavaScript gotchas - http://mistakes.io/#5195547

ha.js

// primitive vs complex numbers
var nrComp = new Number(3);
var nrPrim = Number(3);
nrComp;
nrPrim;
nrComp === nrPrim;
nrComp == nrPrim;
typeof nrComp;
typeof nrPrim;

// inheritance
function Const( veggie ) {
  var veggie = veggie;
  this.fruit = "orange";
  this.get_veggie = function() {
  	return veggie;
  };
}
Const.prototype.spice = "pepper";
typeof Const;
var sub = new Const( "carrot" );
typeof sub;
sub.constructor;
sub.fruit;
sub.veggie;
sub.get_veggie();
sub.spice;
sub.spice = "salt";
sub.spice;
delete sub.spice;
sub.spice; // HA!
delete Const.prototype.spice;
sub.spice;
Const.prototype.spice = "cumin"; // HA!

// object references
var objA = new Object();
objA.tree = "oak";
var objB = objA;
objB.tree;
delete objA.tree;
objA.tree;
objB.tree;
Object.prototype.tree = "fir"; 
objA.tree = "maple";
objB.tree; // HA!

// hoisting - http://jsbin.com/adokem/2/edit
var x = 1;
(function () {
  console.log(x);
  var x = 2;
}());
// vs
function number() {
  return 1;
}
(function () {
  console.log(number()); // function declaration is hoisted
  function number() {
    return 2;
  }
}());
// vs
(function () {
  try {
    number();
  } catch (e) {
    console.log(e.message);
  }
  var number = function number() {
    return 3;
  };
}());