File

The File interface provides information about files and allows JavaScript in a web page to access their content.

File objects are generally retrieved from a FileList object returned as a result of a user selecting files using the <input> element, from a drag and drop operation's DataTransfer object, or from the mozGetAsFile() API on an HTMLCanvasElement. In Gecko, privileged code can create File objects representing any local file without user interaction (see Gecko notes for more information.)

A File object is specific kind of a Blob, and can be used in any context that a Blob can. In particular, FileReader, URL.createObjectURL(), createImageBitmap(), and XMLHttpRequest.send() accept both Blobs and Files.

See Using files from web applications for more information and examples.

Properties

File.lastModifiedDate Read only

The last modified Date of the file referenced by the File object.

File.name Read only

The name of the file referenced by the File object.

The File interface also inherits properties from the Blob interface:

Blob.isClosed Read only

A boolean value, indicating whether the Blob.close() method has been called on the blob. Closed blobs can not be read.

Blob.size Read only

The size, in bytes, of the data contained in the Blob object.

Blob.type Read only

A string indicating the MIME type of the data contained in the Blob. If the type is unknown, this string is empty.

Methods

The File interface doesn't define any methods, but inherits methods from the Blob interface:

Blob.close()

Closes the blob object, possibly freeing underlying resources.

Blob.slice([start[, end[, contentType]]])

Returns a new Blob object containing the data in the specified range of bytes of the source Blob.

Specifications

Browser compatibility

[1] Non-standard implementation.

Implementation notes

• In Gecko, you can use this API from within chrome code. See Using the DOM File API in chrome code for details. To use it from chrome code, JSM and Bootstrap scope, you have to import it using Cu.importGlobalProperties(['File']);
• Starting from Gecko 6.0 (Firefox 6.0 / Thunderbird 6.0 / SeaMonkey 2.3), privileged code (such as extensions) can pass an nsIFile object to the DOM File constructor to specify the file to reference.
• Starting from Gecko 8.0 (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5), you can use new File to create File objects from XPCOM component code instead of having to instantiate the nsIDOMFile object directly. The constructor takes, in contrast to Blob, as second argument the filename. The filename can be any String.

  File File(
    Array parts,
    String filename, 
    BlobPropertyBag properties
  );

• The following non-standard properties and methods were removed in Gecko 7 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4): File.fileName, File.fileSize, File.getAsBinary(), File.getAsDataURL(), File.getAsText(string encoding) (bug 661876). Standard properties File.name, Blob.size, and methods on FileReader should be used instead.

See also

• Using files from web applications
• FileReader
• Using the DOM File API in chrome code (for privileged code running in Gecko, such as Firefox add-ons)