NodeJS async module processing contents of each iteration asynchronously

I am building an application in NodeJS version 8.5 with Express middleware.

I am using the async module to loop over an array of objects. The async.each method iterates over each object and processes each element.

Each object will be processed in a different way depending upon the property.

The way I have it now is to just call a bunch of synchronous methods inside the body of the async.each loop. However, I know that this isn't the 'proper' Node way since everything supposed to be asynchronous.

How can I structure this to use asynchronous methods properly?

async.each(elements_all, (element_original, callback) => {
  // Create new array
  const elements_processed = [];

  // If element has order number property
  if (element_original.order_number) {
    element_original.order_number = processToDisplay(element_original.order_number); //This is a synchronous call
  }

  // If element has po number property
  if (element_original.po_number) {
    element_original.po_number = processToDisplay(element_original.po_number);
  }

  // If element has item price property
  if (element_original.item_price) {
    element_original.item_price = processToDisplay(element_original.item_price);
  }

  // Push to the return array
  entities_processed.push(element_original);

  callback();
}, (err) => { 
  if (err) {
    res.json({
      server_error: true
    });
  } else {
    res.json(entities_processed);
  }
});