JSFiddle - React, Tailwind, and code Playground

HTML

Phrase:
  <input type="text" id="input1" name="LongestWord" placeholder="Put Phrase Here">
  <br>
  <input type="button" id="btn1" value="get Longest Word">
  <br/>
  Longest Word: <span id='sp1'></span>

JavaScript

var btn = document.getElementById("btn1");
var in1 = document.getElementById("input1");
var sp1 = document.getElementById("sp1");
btn.onclick = function(){
    var vals = in1.value.split(' ');
    var val = vals[0];
    vals.forEach(function(v){ if(v.length>val.length) val = v;});
    sp1.textContent = val;
}