How to open a local disk file with JavaScript?

Because I have no life and I want those 4 reputation points so I can show my love to (upvote answers by) people who are actually good at coding I've shared my adaptation of Paolo Moretti's code. Just use openFile(function to be executed with file contents as first parameter).

function dispFile(contents) {
  document.getElementById('contents').innerHTML=contents
}
function clickElem(elem) {
 // Thx user1601638 on Stack Overflow (6/6/2018 - https://stackoverflow.com/questions/13405129/javascript-create-and-save-file )
 var eventMouse = document.createEvent("MouseEvents")
 eventMouse.initMouseEvent("click", true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null)
 elem.dispatchEvent(eventMouse)
}
function openFile(func) {
 readFile = function(e) {
  var file = e.target.files[0];
  if (!file) {
   return;
  }
  var reader = new FileReader();
  reader.onload = function(e) {
   var contents = e.target.result;
   fileInput.func(contents)
   document.body.removeChild(fileInput)
  }
  reader.readAsText(file)
 }
 fileInput = document.createElement("input")
 fileInput.type='file'
 fileInput.style.display='none'
 fileInput.onchange=readFile
 fileInput.func=func
 document.body.appendChild(fileInput)
 clickElem(fileInput)
}

Click the button then choose a file to see its contents displayed below.
<button onclick="openFile(dispFile)">Open a file</button>
<pre id="contents"></pre>

Try

function readFile(file) {
  return new Promise((resolve, reject) => {
    let fr = new FileReader();
    fr.onload = x=> resolve(fr.result);
    fr.readAsText(file);
})}

but user need to take action to choose file

function readFile(file) {
  return new Promise((resolve, reject) => {
    let fr = new FileReader();
    fr.onload = x=> resolve(fr.result);
    fr.readAsText(file);
})}

async function read(input) {
  msg.innerText = await readFile(input.files[0]);
}

<input type="file" onchange="read(this)"/>
<h3>Content:</h3><pre id="msg"></pre>

Others here have given quite elaborate code for this. Perhaps more elaborate code was needed at that time, I don't know. Anyway, I upvoted one of them, but here is a very much simplified version that works the same:

function openFile() {
  document.getElementById('inp').click();
}
function readFile(e) {
  var file = e.target.files[0];
  if (!file) return;
  var reader = new FileReader();
  reader.onload = function(e) {
    document.getElementById('contents').innerHTML = e.target.result;
  }
  reader.readAsText(file)
}

Click the button then choose a file to see its contents displayed below.
<button onclick="openFile()">Open a file</button>
<input id="inp" type='file' style="visibility:hidden;" onchange="readFile(event)" />
<pre id="contents"></pre>

Consider reformatting your file into javascript. Then you can simply load it using good old...

<script src="thefileIwantToLoad.js" defer></script>

Here is how to do it in typescript if Blob is good enough (no need to convert to ByteArray/String via FileReader for many use-cases)

function readFile({
  fileInput,
}: {
  fileInput: HTMLInputElement;
}): Promise<ArrayLike<Blob>> {
  return new Promise((resolve, reject) => {
    fileInput.addEventListener("change", () => {
      resolve(fileInput.files);
    });
  });
}

here is how to do it in vanilla javascript

function readFile({
  fileInput,
}) {
  return new Promise((resolve, reject) => {
    fileInput.addEventListener("change", () => {
      resolve(fileInput.files);
    });
  });
}

It is not related to "security reasons" . And it does not matter if it local or file on network drive. The solution for Windows OS could be IIS - Internet Information Services and this is some details :
To open file in browser with Java Script window.open() , the file should be available on WEB server.
By creating Virtual Directory on your IIS that mapped to any physical drive you should be able to open files. The virtual directory will have some http: address. So instead of window.open("file:///D:/Hello.txt");
You will write window.open("http://iis-server/MY_VIRTUAL_DRIVE_D/Hello.txt");