JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://getfirebug.com/firebug-lite-debug.js"></script>
<input type='file' accept='text/plain' onchange='openFile(event)'>
<br>
<img id='output'>
<script>
  var openFile = function(event) {
    var input = event.target;

    var reader = new FileReader();
    reader.onload = function() {
      var text = reader.result;
      // console.log(reader.result.substring(0, 200));
      // console.log(text);
      var data = JSON.parse(text);
      var features = data.features;

      features.forEach(function(obj, index) {
        obj.geometry.z = obj.attributes.Latitude + obj.attributes.Longtitude;
      });

      //console.log(data); // that's what you want?
      exportData(JSON.stringify(data));

      function exportData(data, name = 'filename.txt') {

        var export_blob = new Blob([data]),
          saveLink = document.createElement('a');
        saveLink.href = window.URL.createObjectURL(export_blob);
        saveLink.download = name;
        saveLink.textContent = 'export';
        document.body.appendChild(saveLink);
      }

    };
    reader.readAsText(input.files[0]);
  };

</script>