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.

The lastModified property of the Document interface returns a string containing the date and local time on which the current document was last modified.

Examples

Simple usage

This example alerts the value of lastModified.

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

Transforming lastModified into a Date object

This example transforms lastModified into a Date object.

let oLastModif = new Date(document.lastModified);

Transforming lastModified into milliseconds

This example transforms lastModified into the number of milliseconds since January 1, 1970, 00:00:00, local time.

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

Note that as a string, lastModified cannot easily be used for comparing the modification dates of documents. Here is a possible example of how to show an 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=${Date.now()}; expires=9999年12月31日 23:59:59 GMT; path=${
 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ドル"));
const lastModif = Date.parse(document.lastModified);
if (Number.isNaN(lastVisit) || lastModif > lastVisit) {
 document.cookie = `last_modif=${Date.now()}; expires=9999年12月31日 23:59:59 GMT; path=${
 location.pathname
 }`;
 if (isFinite(lastVisit)) {
 alert("This page has been changed!");
 }
}

If you want to know whether an external page has changed, you can make a HEAD request using the fetch() API, and examine the Last-Modified response header.