FileReader
The FileReader
object lets web applications asynchronously read the contents of files (or raw data buffers) stored on the user's computer, using File
or Blob
objects to specify the file or data to read.
File objects may be obtained from a FileList
object returned as a result of a user selecting files using the <input>
element, or from a drag and drop operation's DataTransfer
object.
FileReader
can only access the contents of files that the user has explicitly selected, either using an HTML <input type="file">
element or by drag and drop. It cannot be used to read a file by pathname from the user's file system. To read files on the client's file system by pathname, use the File System Access API. To read server-side files, use standard Ajax solutions, with CORS permission if reading cross-domain.
Note: This feature is available in Web Workers
Constructor
FileReader()
-
Returns a new
FileReader
object.
See Using files from web applications for details and examples.
Instance properties
FileReader.error
Read only-
A
DOMException
representing the error that occurred while reading the file. FileReader.readyState
Read only-
A number indicating the state of the
FileReader
. This is one of the following:Name Value Description EMPTY
0
No data has been loaded yet. LOADING
1
Data is currently being loaded. DONE
2
The entire read request has been completed. FileReader.result
Read only-
The file's contents. This property is only valid after the read operation is complete, and the format of the data depends on which of the methods was used to initiate the read operation.
Instance methods
FileReader.abort()
-
Aborts the read operation. Upon return, the
readyState
will beDONE
. FileReader.readAsArrayBuffer()
-
Starts reading the contents of the specified
Blob
, once finished, theresult
attribute contains anArrayBuffer
representing the file's data. FileReader.readAsBinaryString()
-
Starts reading the contents of the specified
Blob
, once finished, theresult
attribute contains the raw binary data from the file as a string. FileReader.readAsDataURL()
-
Starts reading the contents of the specified
Blob
, once finished, theresult
attribute contains adata:
URL representing the file's data. FileReader.readAsText()
-
Starts reading the contents of the specified
Blob
, once finished, theresult
attribute contains the contents of the file as a text string. An optional encoding name can be specified.
Events
Listen to these events using addEventListener()
or by assigning an event listener to the oneventname
property of this interface. Remove the event listeners with removeEventListener()
, once FileReader
is no longer used, to avoid memory leaks.
abort
-
Fired when a read has been aborted, for example because the program called
FileReader.abort()
. error
-
Fired when the read failed due to an error.
load
-
Fired when a read has completed successfully.
loadend
-
Fired when a read has completed, successfully or not.
loadstart
-
Fired when a read has started.
progress
-
Fired periodically as data is read.
Specifications
Specification |
---|
File API # APIASynch |
Browser compatibility
BCD tables only load in the browser