small quiz

 console.log(typeof NaN); // number

console.log(Number([])); // 0

console.log(Number(![])); // 0

console.log(Number([2])); // 2

console.log(Number([0,0])); // NaN

**

undefined means this thing has no value for some reason. Most of the times you encounter undefined it means that something has gone wrong.

On the other hand, null means this thing has no value, I recognise this and it's the way I would like it to be. 看見null表示是編寫者故意設的

typeof(undefined)	Should output ← "undefined"
typeof(null)	Should output ← "object" This is regarded by many as a mistake in the language. It's too late to fix it now so we'll have to deal with it.