Operator dilemma!!

I have questions
1 
console.log(+(1, 2, 3, 4))
// returns undefined
// expected 10😩
  1. why is this valid?
  2. why is the result undefined?
  3. what's happening here?
2

1, 2, 3, 4 is the comma operator - it evaluates to 4, and +4 also evaluates to 4.

console.log() itself always returns undefined, it just prints its argument to the console (4, in this case).

If you want to get 10 from 1, 2, 3, and 4, you can do [1, 2, 3, 4].reduce((a, b) => a + b), for example.

3

See this MDN article on the comma operator.

4

super weird behaviour!