FileReader - Web APIs

FileReader - Web APIs | MDN (2024)

Note: This feature is available in Web Workers.

The FileReader interface 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 fetch(), with CORS permission if reading cross-origin.

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 be DONE.
FileReader.readAsArrayBuffer(): Starts reading the contents of the specified Blob, once finished, the result attribute contains an ArrayBuffer representing the file's data.
FileReader.readAsBinaryString() Deprecated: Starts reading the contents of the specified Blob, once finished, the result attribute contains the raw binary data from the file as a string.
FileReader.readAsDataURL(): Starts reading the contents of the specified Blob, once finished, the result attribute contains a data: URL representing the file's data.
FileReader.readAsText(): Starts reading the contents of the specified Blob, once finished, the result 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

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