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HTML

              
                <div id="fileUploadContainer">
  <h1>Upload a TXT file here</h1>
  <input id="fileUploadInput" type="file" accept=".txt" />
</div>
<hr/>
<div id="fileEditorContainer">
  <h2>Uploaded file content:</h2>
  <input type="text" id="textInput" />
  <br/>
  <button id="generateFile">Generate File</button>
</div>
<hr/>
<div id="downloadButtonContainer" class="hidden">
  <a href="#" id="fileDownload" download>Newly Generated File</a>
</div>


              
            
!

CSS

              
                .hidden{
  display: none;
}
              
            
!

JS

              
                //Declare all elements
const fileInput =   document.getElementById("fileUploadInput");
const textInput = document.getElementById("textInput");
const generateButton = document.getElementById("generateFile");
const downloadButtonContainer = document.getElementById("downloadButtonContainer");
const fileDownload = document.getElementById("fileDownload");

//declare all event Listeners
fileInput.addEventListener("change", handleUpload);
generateButton.addEventListener("click", generateFileDataUrl);

function handleUpload (event){
  //fetch the first file (the element could provide multiple files)
  var file = event.target.files[0];
  var reader = new FileReader();
  
  //use the textInput variable previously declared to update the text input
  reader.onload = (e) => { textInput.value = e.target.result };   

  //trigger the fileReader
  reader.readAsText(file);
}

function generateFileDataUrl(event){
  const inputValue = textInput.value;
  const file = createFileObj(inputValue, "My Text file");
  const reader = new FileReader();
  
  reader.onload = applyDataUrlToAnchor;

  reader.readAsDataURL(file);
}

//create a file with dynamic content
function createFileObj(content, name){
  const file = new File(
    [content],
    `${name}.txt`,
    { type: "text/plain" }
  );
  
  return file;
}

function applyDataUrlToAnchor(e){
  fileDownload.href = e.target.result;
  downloadButtonContainer.classList.remove("hidden");
};
              
            
!
999px

Console