JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="inputfile">
<textarea id="textarea"></textarea>

JavaScript

inputfile.onchange = function () {
  var file = this.files[0],
      reader = new FileReader;

  reader.onloadend = function (evt) {
    textarea.value = reader.result; // file content
  };

  reader.readAsText(file);
};