JSFiddle - React, Tailwind, and code Playground

by some

HTML

<input type='text' id='rounds' value='100000'>
<button type='button' onclick='startTest()'>Start</button>
<ol id='output'></ol>

JavaScript

function get(id) {
  return document.getElementById(id);
}
var test =[
    function test_concat(rounds) {
      var s='';
      for (var idx=0, len=1000*rounds; idx < len; ++idx) {
          s+='Another String '+idx +' ';
      }
      return s;
    },
    function test_join(len) {
      var a=[];
      for (var idx=0; idx < len; ++idx) {
          a.push('Another String ',idx,' ');
      }
      return a.join('');
    }
]

function tJoin() {
}
function startTest() {
    var startTime, endTime, li;
    var out = get('output');
    var rounds = get('rounds').value;
      li = document.createElement('li');
      li.appendChild(
        document.createTextNode(timer(test_array,rounds))
      ); 
      out.appendChild(li);
}
    
    
 
// Javascript
function test_array(size) {
    var s=[];
    for (var i=0; i < size; ++i) { s.push("TEST");}
    return s.join("");
}
function test_string(size) {
    var s="";
    for (var i=0; i < size; ++i) { s += "TEST";}
    return s;
}
function timer(func,size) {
    var start = new Date().getTime();
    return func(size).length + ": " + (new Date().getTime() - start );
}

//var i=100000;

//alert(i + " " + timer(test_array,i));
//i+=100000; alert(i + " " + timer(test_array,i));
//i+=100000; alert(i + " " + timer(test_array,i));