JSFiddle - React, Tailwind, and code Playground
HTML
Escribe algo: <input type = "text" id = "algo" autofocus />
JavaScript
var input = document.getElementById("algo"),
intervalo;
input.addEventListener("keyup", function(){
clearInterval(intervalo);
intervalo = setInterval(function(){
alert ("Has dejado de escribir");
clearInterval(intervalo);
}, 4000);
}, false);