Number([undefined]) -> 0, Number([undefined,undefined]) -> NaN



JavaScript Dev

Number([undefined]) is 0 because JS engine will coerce it which means there will be some abstract operations inside engine and what it does is , it will call toPrimitive(hint), here argument hint means what will be the expected type or what the value should be coerced to. In JS if we try to stringify empty array then it will return empty array removing the brackets and if an array has value undefined/null then it will still return an empty string and if we try to convert empty string to number then we will get 0 as output.

so here what goes in underhood

const arr = [undefined];
// will be now '' and empty string will go to Number('') and we will get 0

when we call toPrimitive and if the hint is number then first it will call valueOf and then toString and if the hint is string then first toString will be called and then valueOf.

Number([undefined,undefined]) is 0 because [undefined, undefined] will be ',' when we coerce it to string and it is not an empty string so we will see NaN and we will see NaN whenever we try to do any mathmetical operation with non string thing.

NaN is an invalid number which will return number when we do typeof because it is a number but invalid.