File: lastModified property

The lastModified read-only property of the {{domxref("File")}} interface provides the last modified date of the file as the number of milliseconds since the Unix epoch (January 1, 1970 at midnight). Files without a known last modified date return the current date.

Value

A number that represents the number of milliseconds since the Unix epoch.

Examples

The example below will loop through the files you choose, and print whether each file was modified within the past year.

HTML

html

<input type="file" id="file-picker" name="fileList" multiple />
<output id="output"></output>

css

output {
  display: block;
  white-space: pre-wrap;
}

JavaScript

const output = document.getElementById("output");
const filePicker = document.getElementById("file-picker");

filePicker.addEventListener("change", (event) => {
  const files = event.target.files;
  const now = new Date();
  output.textContent = "";

  for (const file of files) {
    const date = new Date(file.lastModified);
    // true if the file hasn't been modified for more than 1 year
    const stale = now.getTime() - file.lastModified > 31_536_000_000;
    output.textContent += `${file.name} is ${
      stale ? "stale" : "fresh"
    } (${date}).\n`;
  }
});

Result

Dynamically created files

If a File is created dynamically, the last modified time can be supplied in the {{domxref("File.File()", "File()")}} constructor function. If it is missing, lastModified inherits the current time from {{jsxref("Date.now()")}} at the moment the File object gets created.

const fileWithDate = new File([], "file.bin", {
  lastModified: new Date(2017, 1, 1),
});
console.log(fileWithDate.lastModified); // returns 1485903600000

const fileWithoutDate = new File([], "file.bin");
console.log(fileWithoutDate.lastModified); // returns current time

Reduced time precision

To offer protection against timing attacks and fingerprinting, the precision of someFile.lastModified might get rounded depending on browser settings. In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 2ms. You can also enable privacy.resistFingerprinting, in which case the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

For example, with reduced time precision, the result of someFile.lastModified will always be a multiple of 2, or a multiple of 100 (or privacy.resistFingerprinting.reduceTimerPrecision.microseconds) with privacy.resistFingerprinting enabled.

// reduced time precision (2ms) in Firefox 60
someFile.lastModified;
// Might be:
// 1519211809934
// 1519211810362
// 1519211811670
// …

// reduced time precision with `privacy.resistFingerprinting` enabled
someFile.lastModified;
// Might be:
// 1519129853500
// 1519129858900
// 1519129864400
// …