readTxtFile.js

readTxtFile.js

function readTxtFile(event) {

  return new Promise((resolve, reject) => {
    const file = event.target.files[0];
    const reader = new FileReader();
    reader.onload = () => resolve(reader.result);
    reader.readAsText(file);
  });
}

document.getElementById('files').addEventListener('change', function(e){

  readTxtFile(e)
    .then(res => console.log('then', res))
    .catch(err => console.log('err', err));
}, false);