How to understand undefined and null much better
console.log(undefined + 5) //NaN
console.log(null + 5) //5
console.log(undefined + undefined) //NaN+ operator applies an implicit coercion to an integer to an operand when the other is a number
undefined is coerced to NaN, so console.log(undefined + 5) and console.log(undefined + undefined) output NaN
null is coerced to number 0, so console.log(null + 5) is equivalent to console.log(0 + 5). Therefore 5 is outputted