JSFiddle - React, Tailwind, and code Playground

HTML

<input id="phone" name="phone" type="text" size="15" onblur="foo(this, document.getElementById('phone-help'))">
<span id="phone-help" class="help"></span>

JavaScript

function foo(inputField, helpText) {
  if(inputField.value.length == 0){
    if(helpText !=null){
    	helpText.innerHTML = "Заполните поле";
    	return false;
    }
      
  }
  else {
    if (helpText !=null){
    	 helpText.innerHTML = "";
       return true;
    }
    
  }
 
}