FileReader.readAsText()

readAsText(blob)
readAsText(blob, encoding)

<input type="file" onchange="previewFile()"><br>
<p class="content"></p>

function previewFile() {
  const content = document.querySelector('.content');
  const [file] = document.querySelector('input[type=file]').files;
  const reader = new FileReader();

  reader.addEventListener("load", () => {
    // this will then display a text file
    content.innerText = reader.result;
  }, false);

  if (file) {
    reader.readAsText(file);
  }
}