JSFiddle - React, Tailwind, and code Playground

by George Berkeley

HTML

<form name="formulario">
    <select name="select1" onchange="actualizar()">
      <option>Escoge un número</option>
      <option>uno</option>
      <option>dos</option>
      <option>tres</option>
    </select>

    <select id="select2" name="select2">
      <option>Opciones</option>
      <option>1</option>
      <option>2</option>
      <option>3</option>
    </select>
  </form>

JavaScript

window.onload = function()
  {
	  document.formulario.select2.disabled = true;
  }
  
  
  function actualizar()
  {
	  var indices = document.formulario.select1.selectedIndex;
	  
	  if (indices == 0) x();
	  if (indices == 1) y();
	  
	  document.formulario.select2.disabled = false;
  }

  function x()
  {
	      disabled = document.getElementById("select2").disabled;
		  
	  if (disabled == false)
	  {
		  alert ("SELECT 2 está deshabilitado");
		  document.getElementById("select2").disabled = true;
	  }
  }

  function y()
  {
	  document.getElementById("select2").disabled = false;
  }