Document: lastModified property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

ThelastModified property of theDocumentinterface returns a string containing the date and local time on which the current documentwas last modified.

Examples

Simple usage

This example alerts the value oflastModified.

alert(document.lastModified);// returns: Tuesday, December 16, 2017 11:09:42

Transforming lastModified into a Date object

This example transformslastModified into aDate object.

let oLastModif = new Date(document.lastModified);

Transforming lastModified into milliseconds

This example transformslastModified into the number of milliseconds sinceJanuary 1, 1970, 00:00:00, local time.

let nLastModif = Date.parse(document.lastModified);

Note that as a string,lastModified cannoteasily be used forcomparing the modification dates of documents. Here is a possible example of how to showan alert message when the page changes (see also:JavaScript cookies API):

// Match 'timestamp' in 'last_modif=timestamp'// e.g. '1687964614822' in 'last_modif=1687964614822'const pattern = /last_modif\s*=\s*([^;]*)/;if (  Date.parse(document.lastModified) >  (parseFloat(document.cookie.match(pattern)?.[1]) || 0)) {  document.cookie = `last_modif=${Date.now()}; expires=Fri, 31 Dec 9999 23:59:59 GMT; path=${    location.pathname  }`;  alert("This page has changed!");}

…the same example, but skipping the first visit:

const pattern = /last_modif\s*=\s*([^;]*)/;const lastVisit = parseFloat(document.cookie.replace(pattern, "$1"));const lastModif = Date.parse(document.lastModified);if (Number.isNaN(lastVisit) || lastModif > lastVisit) {  document.cookie = `last_modif=${Date.now()}; expires=Fri, 31 Dec 9999 23:59:59 GMT; path=${    location.pathname  }`;  if (isFinite(lastVisit)) {    alert("This page has been changed!");  }}

If you want to know whether anexternal page has changed, you can make aHEAD request using thefetch() API, and examine theLast-Modified response header.