JSFiddle - React, Tailwind, and code Playground

JavaScript

var mergeTwoLists = function(a, b) {
  if (a === null)
    return b;
  if (b === null)
    return a;
  let result = a;
  if (a.val <= b.val) {
    result.next = mergeTwoLists(a.next, b);
  } else {
    result = b;
    result.next = mergeTwoLists(a, b.next);
  }
  return result;
};

/// START TEST DATA ///
var a = {
  val: 1,
  next: {
    val: 2,
    next: {
      val: 4,
      next: null
    }
  }
};

var b = {
  val: 1,
  next: {
    val: 3,
    next: {
      val: 4,
      next: null
    }
  }
};
///  END TEST DATA ///

var answer = mergeTwoLists(a, b);
while (answer !== null) {
  console.log(answer.val);
  answer = answer.next;
}