let startDate = new Date();
let elapsedTime = 0;
const focus = function() {
startDate = new Date();
};
const blur = function() {
const endDate = new Date();
const spentTime = endDate.getTime() - startDate.getTime();
elapsedTime += spentTime;
};
const beforeunload = function() {
const endDate = new Date();
const spentTime = endDate.getTime() - startDate.getTime();
elapsedTime += spentTime;
// elapsedTime contains the time spent on page in milliseconds
};
window.addEventListener('focus', focus);
window.addEventListener('blur', blur);
window.addEventListener('beforeunload', beforeunload);