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);
  }
}