using let rather than var

CSS

* {
  box-sizing: border-box;
  transition: all 0.2s;
  scroll-behavior: smooth;
}

JS

function run_function() {
  "use strict";
  let x = "This is X";
  let x = "This is not X?";
  write_result(x);
  return x;
}

function run_function_vars() {
  var x = "This is X";
  var x = "This is not X?";
  write_result(x);
  return x;
}

// this is a common function just to write the content into the result div
function write_result(result) {
  var result_html = document.getElementById("result");
  result_html.innerHTML = result;
}

HTML

<div id='result' class='mb-3 alert alert-primary'>..</div>
<button onclick='run_function();' class='btn btn-primary mb-1'>Run Function using let</button>
<button onclick='run_function_vars();' class='btn btn-primary mb-1'>Run Function using var</button>

Working Result

Test Result Below