Either.

Like I could have a recursive solution that returns the sum of the length of the sub-arrays or I could just pass a mutable object and update the length there.

Which way would depend on the exact problem (i.e. Strings would be best collected in a StringBuilder rather than concatenated but `lengthParam` would probably more clear if it returned a number instead of using the Array).

  ```
  const lengthParam = (a,p) => {
    if(Array.isArray(a)) {
      for(let c of a) {
        lengthParam(c,p)
      }
    } else {
      p[0] = p[0] + 1
    }
  };
  
  var b = [0]
  lengthP([1,2,3,[4,5],[6]], b);
  console.log(b[0]) // 6
  ```