JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="" id="mascara" maxlength="6" />

JavaScript

function mascara(o,f){
    v_obj=o
    v_fun=f
    setTimeout("execmascara()",1)
}
function execmascara(){
    v_obj.value=v_fun(v_obj.value)
}
function mask(v){
    v=v.replace(/^(\d{4})(\d)/g,"$1-$2");
    v=v.replace(/(\d)(\d{4})$/,"$1$2");
    return v;
}
function id( el ){
	return document.getElementById( el );
}
window.onload = function(){
	id('mascara').onkeyup = function(){
		mascara( this, mask );
	}
}