JSFiddle - React, Tailwind, and code Playground

HTML

<button id="start">inizia la cattura</button>
<button id="stop">termina la cattura</button>

<span></span>

JavaScript

window.URL = window.URL || window.webkitURL;

var itv, data = new Array(), file,
    start = document.querySelector("#start"),
    stop = document.querySelector("#stop"),
    span = document.querySelector("span");

start.addEventListener('click', init, false);
stop.addEventListener('click', halt, false);

function init() {
    itv = navigator.geolocation.watchPosition(aggiorna);
}

function halt() {
    navigator.geolocation.clearWatch(itv);
    span.innerHTML = JSON.stringify(data);
    file = new Blob([ JSON.stringify(data) ],{ type: "application/json" });
    console.log(file);
}

function aggiorna(pos) {
    data.push({
        lat: pos.coords.latitude,
        lon: pos.coords.longitude,
        acc: pos.coords.accuracy,
        tim: pos.timestamp
    });
}