Find out what is a File object and how to use it

Browsers provide us a File object.

The File object is a Blob object, and it provides 3 properties on top of it:

which adds up to the Blob object properties:

  • size (the size in bytes)
  • type (the MIME type)

You will receive a File object when interacting with the FileList object, which can be retrieved from an HTML Form with an <input type="file" /> element, or when interacting with Drag and Drop.

When you got a FileList object, when you loop over it or you pick an element (for example the first item with myFileList[0]) you will get a File object.

Say you have an input type="file" element in your form:

<input type="file" />

Now you can listen for the change event on this element, so when you choose a file you’ll get information about it. document.querySelector('input').files will return a FileList object, like explained above, and using [0] we get the first file loaded, and we can access all the properties we need from the File object:

document.querySelector('input').addEventListener('change', () => {
  const file = document.querySelector('input').files[0]
  alert(`The file ${} was last modified on ${new Date(file.lastModified).toDateString()}`)

See the Pen File object demo by Flavio Copes (@flaviocopes) on CodePen.