mergeTwoLists.js

/** 
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

var mergeTwoLists = function(l1, l2) {
    const dummy = new ListNode(0);
    let current = dummy;  
  
    while(l1 !== null && l2 !== null){
      if(l1.val <= l2.val){
        current.next = l1;
        l1 = l1.next;        
      } else{
        current.next = l2;
        l2 = l2.next;        
      }
      current = current.next;
    }
  
  	if(l1 !== null){
      current.next = l1;        
    }
      
    if(l2 !== null){
      current.next = l2;
    }
    
    return dummy.next;
};

Thanks it works