JSFiddle - React, Tailwind, and code Playground

JavaScript

function payload(order) {
  return {
    OrderId: order.ORDERID,
    Lines: order.ITEMS.reduce(function(filtered, item) {
      if (item.DispatchedNumber != undefined) {
        var newItem = {
          Qty: item.Qty,
          QtyShipped: item.DispatchedNumber
        }
        
        filtered.push(newItem);
      }
      
      return filtered;
    })
  }
}

test = payload({
  "ORDERID": 1233,
  "ITEMS": [
  	{ Qty: 123, DispatchedNumber: 111 },
    { Qty: 111 },
    { Qty: 444, DispatchedNumber: 555 },
    { Qty: 443, DispatchedNumber: 323 }
  ]
})


console.log(JSON.stringify(test, null, 2));