Closures in Javascript

function add(num) {
    var counter = 0;
    return function (num) {
        counter += num
        return counter
    }
}
  
  const addValue1 = add();
  console.log(addValue1(2)) 
  console.log(addValue1(4))
  console.log(addValue1(1))
  console.log(addValue1(9))
  console.log(addValue1(1))
  const addValue2 = add();
  console.log(addValue2(10))
  console.log(addValue2(20))