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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/Document/lastModified below:

Document: lastModified property - Web APIs

Baseline Widely available

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

A string.

This example alerts the value of lastModified.

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

This example transforms lastModified into a Date object.

let oLastModif = new Date(document.lastModified);

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=${Date.now()}; expires=Fri, 31 Dec 9999 23:59:59 GMT; path=${
    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 an external page has changed, you can make a HEAD request using the fetch() API, and examine the Last-Modified response header.

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