JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="subEmail" onchange="checkFilled();"/>

JavaScript

function checkFilled() {
	var inputVal = document.getElementById("subEmail");
    if (inputVal.value == "") {
        inputVal.style.backgroundColor = "yellow";
    }
    else{
        inputVal.style.backgroundColor = "";
    }
}
 
checkFilled();