Why is the answer of a is 7 and b is 3 in JS?

Question

let a=(1+2,3+4);
console.log(a);
b=1+2,3+4;
console.log(b)

Answer 1

It's called the comma operator
https://m.youtube.com/watch?v=IvqdQB549KQ

Answer 2

Like what?

Answer 3

Thanks Bono bro