FileReader


Purpose

Provides methods to read the File or the Blob objects into memory, and to access the data from those objects with a usage of progress events and event handler attributes.

Result attribute

The result attribute returns the Blob object data in a form of DOMString, ArrayBuffer, or null, depending on the read method that has been called on the FileReader object, and errors that may have occurred. It can also return partial Blob data. Partial Blob data is the part of the File object or the Blob object that has been read into memory currently. When processing the readAsText method, the partial Blob data is a DOMString that is incremented as more bytes are loaded, and when processing the readAsArrayBuffer method the partial Blob data is an ArrayBuffer object instance that consists of the bytes loaded so far.

  • If readyState is 'empty', i.e. no read method has been called, then the result attribute returns null.

  • If an error in reading the File or the Blob objects has occurred with usage of any read method, then the result attribute returns null.

  • If the readAsDataURL method is used, the result attribute returns a DOMString that is a Data URL encoding of the File or the Blob object data.

  • If the readAsText method is called and no error in reading the File or the Blob objects has occurred, then the result attribute returns a string representing the File or the Blob objects data in the format specified by the encoding determination. While processing the readAsText method, this result returns partial Blob data in the format specified by the encoding determination as a DOMString that is incremented as more data is read.

  • If the readAsArrayBuffer method is called and no error in reading the File or the Blob objects has occurred, then the result attribute returns an ArrayBuffer object instance. While processing the readAsArrayBuffer method, the result attribute returns partial Blob data in a form of ArrayBuffer.

Event handler attributes

Event handler attribute Event type Description
onloadstart loadstart When the read starts.
onprogress progress While reading and decoding the Blob, and reporting partial the Blob data (progess.loaded/progress.total).
onabort abort When the read has been aborted. For instance, by invoking the abort method.
onerror error When the read has failed.
onload load When the read has successfully completed.
onloadend loadend When the request has completed: either in success or failure.

FileReader states

The readyState attribute returns the current state of the FileReader object, which takes one of the following values:

State Description Value
Empty The FileReader object has been constructed, and there are no pending reads. None of the read methods have been called. This is the default state of a newly minted FileReader object, until one of the read methods have been called on it. 0
Loading The File or the Blob object is being read. One of the read methods is being processed, and no error has occurred during the read. 1
Done The entire File or Blob object has been read into memory, a file error occurred during the read or the read was aborted using the abort method. If readyState attribute is set to 'Done' it means that at least one of the read methods have been called on this FileReader object. 2

Methods

Name Description
abort

Terminates the algorithm for the currently executed method.

readAsArrayBuffer

Returns the read data as the ArrayBuffer object.

readAsDataURL

Returns the read data as an URL.

readAsText

Returns the read data as a text string.

Related topics

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