Parse JSON array into different Object using Reduce
This can be handled with a single Array.prototype.forEach loop. Consider the following:

function parseItems(json){
  var output = '';
  json.freight.items.forEach(function (item) {
    output += `
      &lt;LineItem&gt;
        &lt;PieceCount&gt;${item.units * item.qty}&lt;/PieceCount&gt;
        &lt;NetWeight UOM="Lb"&gt;${item.weight * item.qty}&lt;/NetWeight&gt;
        &lt;GrossWeight UOM="Lb"&gt;${item.weight * item.qty}&lt;/GrossWeight&gt;
        &lt;FreightClassification&gt;50&lt;/FreightClassification&gt;
      &lt;/LineItem&gt;`;
  });

  return output;
}


No need to iterate over everything repeatedly for this.

Edit: Made a fiddle for you