Variables
+'4' vs parseInt('4')
Numbers
Not all real numbers can accurately be represented in floating point. This can lead to rounding errors. See https://en.wikipedia.org/wiki/Floating-point_arithmetic#Accuracy_problems
Hoisting
console.log(poo);// undefined
var poo = 'far';
console.log(poo);// 'far'
console.log(poo);// undefined
console.log(far);// ReferenceError
var poo = 'far';
Scopes
Block vs Function Scope
var is function scoped, let and const are block scoped.