File: lastModified property
The File.lastModified
read-only property 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="filepicker" name="fileList" multiple />
<output id="output"></output>
JavaScript
js
const output = document.getElementById("output");
const filepicker = document.getElementById("filepicker");
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
new File()
constructor function. If it is missing,
lastModified
inherits the current time from Date.now()
at the
moment the File
object gets created.
js
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 20us in Firefox 59; in 60 it will be 2ms.
js
// reduced time precision (2ms) in Firefox 60
someFile.lastModified;
// 1519211809934
// 1519211810362
// 1519211811670
// …
// reduced time precision with `privacy.resistFingerprinting` enabled
someFile.lastModified;
// 1519129853500
// 1519129858900
// 1519129864400
// …
In Firefox, if you enable privacy.resistFingerprinting
, the
precision will be 100ms or the value of
privacy.resistFingerprinting.reduceTimerPrecision.microseconds
, whichever
is larger.
Specifications
Specification |
---|
File API # dfn-lastModified |
Browser compatibility
BCD tables only load in the browser