JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" value="Hola" /> Hola
<input type="checkbox" value="Mundo" /> Mundo
<textarea></textarea>

CSS

textarea{
  display: block;
}

JavaScript

let out = document.querySelector("textarea"), aux;

document.body.addEventListener("change", function(e){
  if (e.target.type == "checkbox"){
    aux = out.value.split("\n");
    
    if (e.target.checked){
      if (aux.indexOf(e.target.value) < 0){
        out.value += e.target.value + "\n";
      }
    }
    else{
      if (aux.indexOf(e.target.value) > -1){
        aux.splice(aux.indexOf(e.target.value), 1);
        out.value = aux.join("\n");
      }
    }
  }
}, false);