Node.Js promises and async/await - node.js

Question

I am trying to use a oai-pmh harvester to harvest all articles from oai repositor and store them in a local mongo database.
I have a small demo.js file to test and print the titles on the console and it works perfectly. This is my demo.js file:
const oaipmh = require('./my_modules/harvester/harvester.js');
const urlProvider='any OAI URL repo'; // change URL here
let harvester=new oaipmh.Harvester(urlProvider);
harvester.harvest(processItem,function(err,res){
console.log(res);
});
Function processItem(item){
try{
let titles=item.metadata['oai:dc:dc']['dc:title'];
let title=(typeof titles=='object')?titles.join("\n"):titles;
console.log(title);
console.dir(item.metadata['oai_dc:dc']);
}catch(err){
// handle errors here
}
};
I am still trying to learn how to use promises and async/await and how to handle it. So far I have tried to use it inside my controller, but I am getting an error.
On my router file i have:
router.route('/doc')
.get(doc.aggregate_one);
On my controller file i have:
var Doc=require('../models/docModel);
const aoipmh=require('../../harvester/harvester');
exports.aggregate_one = async (req,res,next) => {
const urlProvider = req.header('url');
let harvester=new oaipmh.Harvester(urlProvider);
harveste.harvestAsync(urlProvider,processItemmfunction(err,res){
// do something
});
};
I want to send the URL in the header of my get instead of the barcode. When I am trying to test the GET I am receiving an UnhandledPromiseRejectionWarning: Error; Unexpected status code 400 (expected 200)
How can I use the example of the demo.js file inside my controller in order to handle the Promise?