File: lastModified property
Note: This feature is available in Web Workers.
The lastModified
read-only property of the 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
<input type="file" id="file-picker" name="fileList" multiple />
<output id="output"></output>
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
File()
constructor function. If it is missing,
lastModified
inherits the current time from 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
// …
Specifications
Specification |
---|
File API # dfn-lastModified |
Browser compatibility
BCD tables only load in the browser