JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" size="10" onblur ="foo(this)">
<span id="help"></span>

JavaScript

function foo(inputField){
	var bb = document.getElementById("help");
	if(inputField.value.length==0){
  	
    bb.innerHTML = "Заполните поле"
  }
  else{
  	if(inputField.value.length != null){
      bb.innerHTML = "";
    }
  }
}