JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="txtInput" value='x'/>

JavaScript

var initial = document.getElementById("txtInput").value;

//assigning a change handler to your input
document.getElementById("txtInput").onchange = function() {
    var newx = document.getElementById("txtInput").value;

    if (newx == initial) {
        alert("they match");
    } else {
        alert("no match");
    }
}