var box = document.getElementById("box");

var btn = document.getElementById("btn");

btn.onclick = function(event){

    event.stopPropagation();

    if(box.style.display == "none"){

        box.style.display = "block";

    }else{

        box.style.display = "none";

    }

};

window.onclick = function(){

    box.style.display = "none"

};