Load local files from disk and return blobs, or read them as text, uses promises

localFiles.js

function loadFile() {
  const input = document.createElement("input");
  input.type = "file";
  input.click();
  return new Promise(function(resolve, reject) {
    input.onchange = function() {
      resolve( input.files[0] );
    };
  });
}

function readFile(blob, readAs="readAsText") {
  const reader = new FileReader();
  reader[readAs](blob);
  return new Promise(function(resolve, reject) {
    reader.onloadend = function() {
      resolve( reader.result );
    };
  });
}

// Wrapper
function openFile() {
  return loadFile().then(readFile);
}

// Example
openFile().then(console.log);