File API

1. Introduction

This section is informative.

Web applications should have the ability to manipulate as wide as possible a range of user input, including files that a user may wish to upload to a remote server or manipulate inside a rich web application. This specification defines the basic representations for files, lists of files, errors raised by access to files, and programmatic ways to read files. Additionally, this specification also defines an interface that represents "raw data" which can be asynchronously processed on the main thread of conforming user agents. The interfaces and API defined in this specification can be used with other interfaces and APIs exposed to the web platform.

The File interface represents file data typically obtained from the underlying file system, and the Blob interface ("Binary Large Object" - a name originally introduced to web APIs in Google Gears) represents immutable raw data. File or Blob reads should happen asynchronously on the main thread, with an optional synchronous API used within threaded web applications. An asynchronous API for reading files prevents blocking and UI "freezing" on a user agent’s main thread. This specification defines an asynchronous API based on an event model to read and access a File or Blob’s data. A FileReader object provides asynchronous read methods to access that file’s data through event handler content attributes and the firing of events. The use of events and event handlers allows separate code blocks the ability to monitor the progress of the read (which is particularly useful for remote drives or mounted drives, where file access performance may vary from local drives) and error conditions that may arise during reading of a file. An example will be illustrative.

function startRead() {
  // obtain input element through DOM

  var file = document.getElementById('file').files[0];
  if(file){
    getAsText(file);
  }
}

function getAsText(readFile) {

  var reader = new FileReader();

  // Read file into memory as UTF-16
  reader.readAsText(readFile, "UTF-16");

  // Handle progress, success, and errors
  reader.onprogress = updateProgress;
  reader.onload = loaded;
  reader.onerror = errorHandler;
}

function updateProgress(evt) {
  if (evt.lengthComputable) {
    // evt.loaded and evt.total are ProgressEvent properties
    var loaded = (evt.loaded / evt.total);
    if (loaded < 1) {
      // Increase the prog bar length
      // style.width = (loaded * 200) + "px";
    }
  }
}

function loaded(evt) {
  // Obtain the read file data
  var fileString = evt.target.result;
  // Handle UTF-16 file dump
  if(utils.regexp.isChinese(fileString)) {
    //Chinese Characters + Name validation
  }
  else {
    // run other charset test
  }
  // xhr.send(fileString)
}

function errorHandler(evt) {
  if(evt.target.error.name == "NotReadableError") {
    // The file could not be read
  }
}

2. Terminology and Algorithms

When this specification says to terminate an algorithm the user agent must terminate the algorithm after finishing the step it is on. Asynchronous read methods defined in this specification may return before the algorithm in question is terminated, and can be terminated by an abort() call.

The algorithms and steps in this specification use the following mathematical operations:

• max(a,b) returns the maximum of a and b, and is always performed on integers as they are defined in WebIDL [WebIDL]; in the case of max(6,4) the result is 6. This operation is also defined in ECMAScript [ECMA-262].

• min(a,b) returns the minimum of a and b, and is always performed on integers as they are defined in WebIDL [WebIDL]; in the case of min(6,4) the result is 4. This operation is also defined in ECMAScript [ECMA-262].

• Mathematical comparisons such as < (less than), ≤ (less than or equal to), and > (greater than) are as in ECMAScript [ECMA-262].

The term Unix Epoch is used in this specification to refer to the time 00:00:00 UTC on January 1 1970 (or 1970-01-01T00:00:00Z ISO 8601); this is the same time that is conceptually "0" in ECMA-262 [ECMA-262].

3. The Blob Interface and Binary Data

A Blob object refers to a byte sequence, and has a size attribute which is the total number of bytes in the byte sequence, and a type attribute, which is an ASCII-encoded string in lower case representing the media type of the byte sequence.

Each Blob must have an internal snapshot state, which must be initially set to the state of the underlying storage, if any such underlying storage exists. Further normative definition of snapshot state can be found for Files.

[Constructor(optional sequence<BlobPart> blobParts,
             optional BlobPropertyBag options),
 Exposed=(Window,Worker), Serializable]
interface Blob {

  readonly attribute unsigned long long size;
  readonly attribute DOMString type;

  // slice Blob into byte-ranged chunks
  Blob slice([Clamp] optional long long start,
            [Clamp] optional long long end,
            optional DOMString contentType);
};

enum EndingType { "transparent", "native" };

dictionary BlobPropertyBag {
  DOMString type = "";
  EndingType endings = "transparent";
};

typedef (BufferSource or Blob or USVString) BlobPart;

Blob objects are serializable objects. Their serialization steps, given value and serialized, are:

1. Set serialized.[[SnapshotState]] to value’s snapshot state.

2. Set serialized.[[ByteSequence]] to value’s underlying byte sequence.

Their deserialization step, given serialized and value, are:

1. Set value’s snapshot state to serialized.[[SnapshotState]].

2. Set value’s underlying byte sequence to serialized.[[ByteSequence]].

3.1. Constructors

3.1.1. Constructor Parameters

The Blob() constructor can be invoked with the parameters below:

A blobParts sequence

which takes any number of the following types of elements, and in any order:

• BufferSource elements.

• Blob elements.

• USVString elements.

An optional BlobPropertyBag

which takes these optional members:

• type, the ASCII-encoded string in lower case representing the media type of the Blob. Normative conditions for this member are provided in the §3.1 Constructors.

• endings, an enum which can take the values "transparent" or "native". By default this is set to "transparent". If set to "native", line endings will be converted to native in any USVString elements in blobParts.

// Create a new Blob object

var a = new Blob();

// Create a 1024-byte ArrayBuffer
// buffer could also come from reading a File

var buffer = new ArrayBuffer(1024);

// Create ArrayBufferView objects based on buffer

var shorts = new Uint16Array(buffer, 512, 128);
var bytes = new Uint8Array(buffer, shorts.byteOffset + shorts.byteLength);

var b = new Blob(["foobarbazetcetc" + "birdiebirdieboo"], {type: "text/plain;charset=utf-8"});

var c = new Blob([b, shorts]);

var a = new Blob([b, c, bytes]);

var d = new Blob([buffer, b, c, bytes]);

3.2. Attributes

size, of type unsigned long long, readonly

Returns the size of the byte sequence in number of bytes. On getting, conforming user agents must return the total number of bytes that can be read by a FileReader or FileReaderSync object, or 0 if the Blob has no bytes to be read.

type, of type DOMString, readonly

The ASCII-encoded string in lower case representing the media type of the Blob. On getting, user agents must return the type of a Blob as an ASCII-encoded string in lower case, such that when it is converted to a byte sequence, it is a parsable MIME type, or the empty string – 0 bytes – if the type cannot be determined.

The type attribute can be set by the web application itself through constructor invocation and through the slice() call; in these cases, further normative conditions for this attribute are in §3.1 Constructors, §4.1 Constructor, and §3.3.1 The slice method respectively. User agents can also determine the type of a Blob, especially if the byte sequence is from an on-disk file; in this case, further normative conditions are in the file type guidelines.

Note: The type t of a Blob is considered a parsable MIME type, if performing the parse a MIME type algorithm to a byte sequence converted from the ASCII-encoded string representing the Blob object’s type does not return undefined.

Note: Use of the type attribute informs the encoding determination and determines the Content-Type header when fetching blob URLs.

3.3. Methods and Parameters

3.3.1. The slice method

The slice() method returns a new Blob object with bytes ranging from the optional start parameter up to but not including the optional end parameter, and with a type attribute that is the value of the optional contentType parameter. It must act as follows:

1. Let O be the Blob context object on which the slice() method is being called.

2. The optional start parameter is a value for the start point of a slice() call, and must be treated as a byte-order position, with the zeroth position representing the first byte. User agents must process slice() with start normalized according to the following:

   1. If the optional start parameter is not used as a parameter when making this call, let relativeStart be 0.
   2. If start is negative, let relativeStart be max((size + start), 0).
   3. Else, let relativeStart be min(start, size).

3. The optional end parameter is a value for the end point of a slice() call. User agents must process slice() with end normalized according to the following:

   1. If the optional end parameter is not used as a parameter when making this call, let relativeEnd be size.
   2. If end is negative, let relativeEnd be max((size + end), 0).
   3. Else, let relativeEnd be min(end, size).

4. The optional contentType parameter is used to set the ASCII-encoded string in lower case representing the media type of the Blob. User agents must process the slice() with contentType normalized according to the following:

   1. If the contentType parameter is not provided, let relativeContentType be set to the empty string.
   2. Else let relativeContentType be set to contentType and run the substeps below:

      1. If relativeContentType contains any characters outside the range of U+0020 to U+007E, then set relativeContentType to the empty string and return from these substeps.

      2. Convert every character in relativeContentType to ASCII lowercase.

5. Let span be max((relativeEnd - relativeStart), 0).

6. Return a new Blob object S with the following characteristics:

   1. S refers to span consecutive bytes from O, beginning with the byte at byte-order position relativeStart.
   2. S.size = span.
   3. S.type = relativeContentType.

// obtain input element through DOM

var file = document.getElementById('file').files[0];
if(file)
{
  // create an identical copy of file
  // the two calls below are equivalent

  var fileClone = file.slice();
  var fileClone2 = file.slice(0, file.size);

  // slice file into 1/2 chunk starting at middle of file
  // Note the use of negative number

  var fileChunkFromEnd = file.slice(-(Math.round(file.size/2)));

  // slice file into 1/2 chunk starting at beginning of file

  var fileChunkFromStart = file.slice(0, Math.round(file.size/2));

  // slice file from beginning till 150 bytes before end

  var fileNoMetadata = file.slice(0, -150, "application/experimental");
}

4. The File Interface

A File object is a Blob object with a name attribute, which is a string; it can be created within the web application via a constructor, or is a reference to a byte sequence from a file from the underlying (OS) file system.

If a File object is a reference to a byte sequence originating from a file on disk, then its snapshot state should be set to the state of the file on disk at the time the File object is created.

Note: This is a non-trivial requirement to implement for user agents, and is thus not a must but a should [RFC2119]. User agents should endeavor to have a File object’s snapshot state set to the state of the underlying storage on disk at the time the reference is taken. If the file is modified on disk following the time a reference has been taken, the File's snapshot state will differ from the state of the underlying storage. User agents may use modification time stamps and other mechanisms to maintain snapshot state, but this is left as an implementation detail.

When a File object refers to a file on disk, user agents must return the type of that file, and must follow the file type guidelines below:

• User agents must return the type as an ASCII-encoded string in lower case, such that when it is converted to a corresponding byte sequence, it is a parsable MIME type, or the empty string – 0 bytes – if the type cannot be determined.

• When the file is of type text/plain user agents must NOT append a charset parameter to the dictionary of parameters portion of the media type [MIMESNIFF].

• User agents must not attempt heuristic determination of encoding, including statistical methods.

  [Constructor(sequence<BlobPart> fileBits,
               USVString fileName,
               optional FilePropertyBag options),
   Exposed=(Window,Worker), Serializable]
  interface File : Blob {
    readonly attribute DOMString name;
    readonly attribute long long lastModified;
  };
  
  dictionary FilePropertyBag : BlobPropertyBag {
    long long lastModified;
  };
  

File objects are serializable objects. Their serialization steps, given value and serialized, are:

1. Set serialized.[[SnapshotState]] to value’s snapshot state.

2. Set serialized.[[ByteSequence]] to value’s underlying byte sequence.

3. Set serialized.[[Name]] to the value of value’s name attribute.

4. Set serialized.[[LastModified]] to the value of value’s lastModified attribute.

Their deserialization steps, given value and serialized, are:

1. Set value’s snapshot state to serialized.[[SnapshotState]].

2. Set value’s underlying byte sequence to serialized.[[ByteSequence]].

3. Initialize the value of value’s name attribute to serialized.[[Name]].

4. Initialize the value of value’s lastModified attribute to serialized.[[LastModified]].

4.1. Constructor

4.1.1. Constructor Parameters

The File() constructor can be invoked with the parameters below:

A fileBits sequence

which takes any number of the following elements, and in any order:

• BufferSource elements.

• Blob elements, which includes File elements.

• USVString elements.

A fileName parameter

A USVString parameter representing the name of the file; normative conditions for this constructor parameter can be found in §4.1 Constructor.

An optional FilePropertyBag dictionary

which in addition to the members of BlobPropertyBag takes one member:

• An optional lastModified member, which must be a long long; normative conditions for this member are provided in §4.1 Constructor.

4.2. Attributes

name, of type DOMString, readonly

The name of the file. On getting, this must return the name of the file as a string. There are numerous file name variations and conventions used by different underlying OS file systems; this is merely the name of the file, without path information. On getting, if user agents cannot make this information available, they must return the empty string. If a File object is created using a constructor, further normative conditions for this attribute are found in §4.1 Constructor.

lastModified, of type long long, readonly

The last modified date of the file. On getting, if user agents can make this information available, this must return a long long set to the time the file was last modified as the number of milliseconds since the Unix Epoch. If the last modification date and time are not known, the attribute must return the current date and time as a long long representing the number of milliseconds since the Unix Epoch; this is equivalent to Date.now() [ECMA-262]. If a File object is created using a constructor, further normative conditions for this attribute are found in §4.1 Constructor.

The File interface is available on objects that expose an attribute of type FileList; these objects are defined in HTML [HTML]. The File interface, which inherits from Blob, is immutable, and thus represents file data that can be read into memory at the time a read operation is initiated. User agents must process reads on files that no longer exist at the time of read as errors, throwing a NotFoundError exception if using a FileReaderSync on a Web Worker [Workers] or firing an error event with the error attribute returning a NotFoundError.

var file = document.getElementById("filePicker").files[0];
var date = new Date(file.lastModified);
println("You selected the file " + file.name + " which was modified on " + date.toDateString() + ".");

...

// Generate a file with a specific last modified date

var d = new Date(2013, 12, 5, 16, 23, 45, 600);
var generatedFile = new File(["Rough Draft ...."], "Draft1.txt", {type: "text/plain", lastModified: d})

...

5. The FileList Interface

Note: The FileList interface should be considered "at risk" since the general trend on the Web Platform is to replace such interfaces with the Array platform object in ECMAScript [ECMA-262]. In particular, this means syntax of the sort filelist.item(0) is at risk; most other programmatic use of FileList is unlikely to be affected by the eventual migration to an Array type.

This interface is a list of File objects.

[Exposed=(Window,Worker), Serializable]
interface FileList {
  getter File? item(unsigned long index);
  readonly attribute unsigned long length;
};

FileList objects are serializable objects. Their serialization steps, given value and serialized, are:

1. Set serialized.[[Files]] to an empty list.

2. For each file in value, append the sub-serialization of file to serialized.[[Files]].

Their deserialization step, given serialized and value, are:

1. For each file of serialized.[[Files]], add the sub-deserialization of file to value.

// uploadData is a form element
// fileChooser is input element of type 'file'
var file = document.forms['uploadData']['fileChooser'].files[0];

// alternative syntax can be
// var file = document.forms['uploadData']['fileChooser'].files.item(0);

if(file)
{
  // Perform file ops
}

5.1. Attributes

length, of type unsigned long, readonly

must return the number of files in the FileList object. If there are no files, this attribute must return 0.

5.2. Methods and Parameters

item(index)

must return the indexth File object in the FileList. If there is no indexth File object in the FileList, then this method must return null.

index must be treated by user agents as value for the position of a File object in the FileList, with 0 representing the first file. Supported property indices are the numbers in the range zero to one less than the number of File objects represented by the FileList object. If there are no such File objects, then there are no supported property indices.

Note: The HTMLInputElement interface has a readonly attribute of type FileList, which is what is being accessed in the above example. Other interfaces with a readonly attribute of type FileList include the DataTransfer interface.

6. Reading Data

6.1. The Read Operation

The algorithm below defines a read operation, which takes a Blob and a synchronous flag as input, and reads bytes into a byte stream which is returned as the result of the read operation, or else fails along with a failure reason. Methods in this specification invoke the read operation with the synchronous flag either set or unset.

The synchronous flag determines if a read operation is synchronous or asynchronous, and is unset by default. Methods may set it. If it is set, the read operation takes place synchronously. Otherwise, it takes place asynchronously.

To perform a read operation on a Blob and the synchronous flag, run the following steps:

1. Let s be a a new body, b be the Blob to be read from, and bytes initially set to an empty byte sequence. Set the length on s to the size of b. While there are still bytes to be read in b, perform the following substeps:

   1. If the synchronous flag is set, follow the steps below:

      1. Let bytes be the byte sequence that results from reading a chunk from b. If a file read error occurs reading a chunk from b, return s with the error flag set, along with a failure reason, and terminate this algorithm.

         Note: Along with returning failure, the synchronous part of this algorithm must return the failure reason that occurred for throwing an exception by synchronous methods that invoke this algorithm with the synchronous flag set.

      2. If there are no errors, push bytes to s, and increment s’s transmitted [Fetch] by the number of bytes in bytes. Reset bytes to the empty byte sequence and continue reading chunks as above.

      3. When all the bytes of b have been read into s, return s and terminate this algorithm.

   2. Otherwise, the synchronous flag is unset. Return s and process the rest of this algorithm asynchronously.

   3. Let bytes be the byte sequence that results from reading a chunk from b. If a file read error occurs reading a chunk from b, set the error flag on s, and terminate this algorithm with a failure reason.

      Note: The asynchronous part of this algorithm must signal the failure reason that occurred for asynchronous error reporting by methods expecting s and which invoke this algorithm with the synchronous flag unset.

   4. If no file read error occurs, push bytes to s, and increment s’s transmitted [Fetch] by the number of bytes in bytes. Reset bytes to the empty byte sequence and continue reading chunks as above.

To perform an annotated task read operation on a Blob b, perform the steps below:

1. Perform a read operation on b with the synchronous flag unset, along with the additional steps below.

2. If the read operation terminates with a failure reason, queue a task to process read error with the failure reason and terminate this algorithm.

3. When the first chunk is being pushed to the body s during the read operation, queue a task to process read.

4. Once the body s from the read operation has at least one chunk read into it, or there are no chunks left to read from b, queue a task to process read data. Keep queuing tasks to process read data for every chunk read or every 50ms, whichever is least frequent.

5. When all of the chunks from b are read into the body s from the read operation, queue a task to process read EOF.

Use the file reading task source for all these tasks.

6.2. The File Reading Task Source

This specification defines a new generic task source called the file reading task source, which is used for all tasks that are queued in this specification to read byte sequences associated with Blob and File objects. It is to be used for features that trigger in response to asynchronously reading binary data.

6.3. The FileReader API

[Constructor, Exposed=(Window,Worker)]
interface FileReader: EventTarget {

  // async read methods
  void readAsArrayBuffer(Blob blob);
  void readAsBinaryString(Blob blob);
  void readAsText(Blob blob, optional DOMString label);
  void readAsDataURL(Blob blob);

  void abort();

  // states
  const unsigned short EMPTY = 0;
  const unsigned short LOADING = 1;
  const unsigned short DONE = 2;


  readonly attribute unsigned short readyState;

  // File or Blob data
  readonly attribute (DOMString or ArrayBuffer)? result;

  readonly attribute DOMException? error;

  // event handler content attributes
  attribute EventHandler onloadstart;
  attribute EventHandler onprogress;
  attribute EventHandler onload;
  attribute EventHandler onabort;
  attribute EventHandler onerror;
  attribute EventHandler onloadend;

};

6.3.1. Constructor

When the FileReader() constructor is invoked, the user agent must return a new FileReader object.

In environments where the global object is represented by a Window or a WorkerGlobalScope object, the FileReader constructor must be available.

6.3.2. Event Handler Content Attributes

The following are the event handler content attributes (and their corresponding event handler event types) that user agents must support on FileReader as DOM attributes:

6.3.3. FileReader States

The FileReader object can be in one of 3 states. The readyState attribute, on getting, must return the current state, which must be one of the following values:

EMPTY (numeric value 0)

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.

LOADING (numeric value 1)

A File or Blob is being read. One of the read methods is being processed, and no error has occurred during the read.

DONE (numeric value 2)

The entire File or Blob has been read into memory, OR a file read error occurred, OR the read was aborted using abort(). The FileReader is no longer reading a File or Blob. If readyState is set to DONE it means at least one of the read methods have been called on this FileReader.

6.3.4. Reading a File or Blob

The FileReader interface makes available several asynchronous read methods—readAsArrayBuffer(), readAsBinaryString(), readAsText() and readAsDataURL(), which read files into memory. If multiple concurrent read methods are called on the same FileReader object, user agents must throw an InvalidStateError on any of the read methods that occur when readyState = LOADING.

(FileReaderSync makes available several synchronous read methods. Collectively, the sync and async read methods of FileReader and FileReaderSync are referred to as just read methods.)

6.3.4.1. The result attribute

On getting, the result attribute returns a Blob's data as a DOMString, or as an ArrayBuffer, or null, depending on the read method that has been called on the FileReader, and any errors that may have occurred.

The list below is normative for the result attribute and is the conformance criteria for this attribute:

• On getting, if the readyState is EMPTY (no read method has been called) then the result attribute must return null.

• On getting, if an error in reading the File or Blob has occurred (using any read method) then the result attribute must return null.

• On getting, if the readAsDataURL() read method is used, the result attribute must return a DOMString that is a Data URL [RFC2397] encoding of the File or Blob's data.

• On getting, if the readAsBinaryString() read method is called and no error in reading the File or Blob has occurred, then the result attribute must return a DOMString representing the File or Blob's data as a binary string, in which every byte is represented by a code unit of equal value [0...255].

• On getting, if the readAsText() read method is called and no error in reading the File or Blob has occurred, then the result attribute must return a string representing the File or Blob's data as a text string, and should decode the string into memory in the format specified by the encoding determination as a DOMString.

• On getting, if the readAsArrayBuffer() read method is called and no error in reading the File or Blob has occurred, then the result attribute must return an ArrayBuffer object.

6.3.4.2. The readAsDataURL() method

When the readAsDataURL(blob) method is called, the user agent must run the steps below.

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise set readyState to LOADING.

3. Initiate an annotated task read operation using the blob argument as input and handle tasks queued on the file reading task source per below.

4. To process read error with a failure reason, proceed to §6.3.4.6 Error Steps.

5. To process read fire a progress event called loadstart at the context object.

6. To process read data fire a progress event called progress at the context object.

7. To process read EOF run these substeps:

   1. Set readyState to DONE.

   2. Set the result attribute to the body returned by the read operation as a DataURL [RFC2397]; on getting, the result attribute returns the blob as a Data URL [RFC2397].

      • Use the blob’s type attribute as part of the Data URL if it is available in keeping with the Data URL specification [RFC2397].

      • If the type attribute is not available on the blob return a Data URL without a media-type. [RFC2397]. Data URLs that do not have media-types [RFC2046] must be treated as plain text by conforming user agents. [RFC2397].

   3. Fire a progress event called load at the context object.

   4. Unless readyState is LOADING fire a progress event called loadend at the context object. If readyState is LOADING do NOT fire loadend at the context object.

8. Terminate this algorithm.

6.3.4.3. The readAsText() method

The readAsText() method can be called with an optional parameter, label, which is a DOMString argument that represents the label of an encoding [Encoding]; if provided, it must be used as part of the encoding determination used when processing this method call.

When the readAsText(blob, label) method is called, the user agent must run the steps below.

1. If readyState = LOADING throw an InvalidStateError and terminate this algorithm.

2. Otherwise set readyState to LOADING.

3. Initiate an annotated task read operation using the blob argument as input and handle tasks queued on the file reading task source per below.

4. To process read error with a failure reason, proceed to the §6.3.4.6 Error Steps.

5. To process read fire a progress event called loadstart at the context object.

6. To process read data fire a progress event called progress at the context object.

7. To process read EOF run these substeps:

   1. Set readyState to DONE

   2. Set the result attribute to the body returned by the read operation, represented as a string in a format determined by the encoding determination.

   3. Fire a progress event called load at the context object.

   4. Unless readyState is LOADING fire a progress event called loadend at the context object. If readyState is LOADING do NOT fire loadend at the context object.

8. Terminate this algorithm.

6.3.4.4. The readAsArrayBuffer() method

When the readAsArrayBuffer(blob) method is called, the user agent must run the steps below.

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise set readyState to LOADING.

3. Initiate an annotated task read operation using the blob argument as input and handle tasks queued on the file reading task source per below.

4. To process read error with a failure reason, proceed to the §6.3.4.6 Error Steps.

5. To process read fire a progress event called loadstart at the context object.

6. To process read data fire a progress event called progress at the context object.

7. To process read EOF run these substeps:

   1. Set readyState to DONE

   2. Set the result attribute to the body returned by the read operation as an ArrayBuffer object.

   3. Fire a progress event called load at the context object.

   4. Unless readyState is LOADING fire a progress event called loadend at the context object. If readyState is LOADING do NOT fire loadend at the context object.

8. Terminate this algorithm.

6.3.4.5. The readAsBinaryString() method

When the readAsBinaryString(blob) method is called, the user agent must run the steps below.

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise set readyState to LOADING.

3. Initiate an annotated task read operation using the blob argument as input and handle tasks queued on the file reading task source per below.

4. To process read error with a failure reason, proceed to the §6.3.4.6 Error Steps.

5. To process read fire a progress event called loadstart at the context object.

6. To process read data fire a progress event called progress at the context object.

7. To process read EOF run these substeps:

   1. Set readyState to DONE

   2. Set the result attribute to the body returned by the read operation as a binary string.

   3. Fire a progress event called load at the context object.

   4. Unless readyState is LOADING fire a progress event called loadend at the context object. If readyState is LOADING do NOT fire loadend at the context object.

8. Terminate this algorithm.

6.3.4.6. Error Steps

These error steps are to process read error with a failure reason.

1. Set the context object’s readyState to DONE and result to null if it is not already set to null.

2. Set the error attribute on the context object; on getting, the error attribute must be a a DOMException object that corresponds to the failure reason. Fire a progress event called error at the context object.

3. Unless readyState is LOADING, fire a progress event called loadend at the context object. If readyState is LOADING do NOT fire loadend at the context object.

4. Terminate the algorithm for any read method.

6.3.4.7. The abort() method

When the abort() method is called, the user agent must run the steps below:

1. If readyState = EMPTY or if readyState = DONE set result to null and terminate this algorithm.

2. If readyState = LOADING set readyState to DONE and result to null.

3. If there are any tasks from the context object on the file reading task source in an affiliated task queue, then remove those tasks from that task queue.

4. Terminate the algorithm for the read method being processed.

5. Fire a progress event called abort.

6. Fire a progress event called loadend.

6.3.4.8. Blob Parameters

The asynchronous read methods, the synchronous read methods, and URL.createObjectURL() take a Blob parameter. This section defines this parameter.

blob

This is a Blob argument and must be a reference to a single File in a FileList or a Blob argument not obtained from the underlying OS file system.

6.4. Determining Encoding

When reading Blob objects using the readAsText() read method, the following encoding determination steps must be followed:

1. Let encoding be null.

2. If the label argument is present when calling the method, set encoding to the result of the getting an encoding from label.

3. If the getting an encoding steps above return failure, then set encoding to null.

4. If encoding is null, and the blob argument’s type attribute is present, and it uses a Charset Parameter [RFC2046], set encoding to the result of getting an encoding for the portion of the Charset Parameter that is a label of an encoding.

   If blob has a type attribute of text/plain;charset=utf-8 then getting an encoding is run using "utf-8" as the label. Note that user agents must parse and extract the portion of the Charset Parameter that constitutes a label of an encoding.

5. If the getting an encoding steps above return failure, then set encoding to null.

6. If encoding is null, then set encoding to utf-8.

7. Decode this blob using fallback encoding encoding, and return the result. On getting, the result attribute of the FileReader object returns a string in encoding format. The synchronous readAsText() method of the FileReaderSync object returns a string in encoding format.

6.5. Events

The FileReader object must be the event target for all events in this specification.

When this specification says to fire a progress event called e (for some ProgressEvent e at a given FileReader reader as the context object), the following are normative:

• The progress event e does not bubble. e.bubbles must be false [DOM]

• The progress event e is NOT cancelable. e.cancelable must be false [DOM]

6.5.1. Event Summary

The following are the events that are fired at FileReader objects.

6.5.2. Summary of Event Invariants

This section is informative.

The following are invariants applicable to event firing for a given asynchronous read method in this specification:

1. Once a loadstart has been fired, a corresponding loadend fires at completion of the read, UNLESS any of the following are true:

   • the read method has been cancelled using abort() and a new read method has been invoked

   • the event handler function for a load event initiates a new read

   • the event handler function for a error event initiates a new read.

   Note: The events loadstart and loadend are not coupled in a one-to-one manner.

   This example showcases "read-chaining": initiating another read from within an event handler while the "first" read continues processing.

   // In code of the sort...
   reader.readAsText(file);
   reader.onload = function(){reader.readAsText(alternateFile);}
   
   .....
   
   //... the loadend event must not fire for the first read
   
   reader.readAsText(file);
   reader.abort();
   reader.onabort = function(){reader.readAsText(updatedFile);}
   
   //... the loadend event must not fire for the first read
   

2. One progress event will fire when blob has been completely read into memory.

3. No progress event fires before loadstart.

4. No progress event fires after any one of abort, load, and error have fired. At most one of abort, load, and error fire for a given read.

5. No abort, load, or error event fires after loadend.

6.6. Reading on Threads

Web Workers allow for the use of synchronous File or Blob read APIs, since such reads on threads do not block the main thread. This section defines a synchronous API, which can be used within Workers [[Web Workers]]. Workers can avail of both the asynchronous API (the FileReader object) and the synchronous API (the FileReaderSync object).

6.6.1. The FileReaderSync API

This interface provides methods to synchronously read File or Blob objects into memory.

[Constructor, Exposed=(DedicatedWorker,SharedWorker)]
interface FileReaderSync {
  // Synchronously return strings

  ArrayBuffer readAsArrayBuffer(Blob blob);
  DOMString readAsBinaryString(Blob blob);
  DOMString readAsText(Blob blob, optional DOMString label);
  DOMString readAsDataURL(Blob blob);
};

6.6.1.1. Constructors

When the FileReaderSync() constructor is invoked, the user agent must return a new FileReaderSync object.

In environments where the global object is represented by a WorkerGlobalScope object, the FileReaderSync constructor must be available.

6.6.1.2. The readAsText() method

When the readAsText(blob, label) method is called, the following steps must be followed:

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise, initiate a read operation using the blob argument, and with the synchronous flag set. If the read operation returns failure, throw the appropriate exception as defined in §7.1 Throwing an Exception or Returning an Error. Terminate this algorithm.

3. If no error has occurred, return the result of the read operation represented as a string in a format determined through the encoding determination algorithm.

6.6.1.3. The readAsDataURL() method

When the readAsDataURL(blob) method is called, the following steps must be followed:

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise, initiate a read operation using the blob argument, and with the synchronous flag set. If the read operation returns failure, throw the appropriate exception as defined in §7.1 Throwing an Exception or Returning an Error. Terminate this algorithm.

3. If no error has occurred, return the result of the read operation as a Data URL [RFC2397] subject to the considerations below:

   • Use the blob’s type attribute as part of the Data URL if it is available in keeping with the Data URL specification [RFC2397].

   • If the type attribute is not available on the blob return a Data URL without a media-type. [RFC2397]. Data URLs that do not have media-types [RFC2046] must be treated as plain text by conforming user agents. [RFC2397].

6.6.1.4. The readAsArrayBuffer() method

When the readAsArrayBuffer(blob) method is called, the following steps must be followed:

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise, initiate a read operation using the blob argument, and with the synchronous flag set. If the read operation returns failure, throw the appropriate exception as defined in §7.1 Throwing an Exception or Returning an Error. Terminate this algorithm.

3. If no error has occurred, return the result of the read operation as an ArrayBuffer.

6.6.1.5. The readAsBinaryString() method

When the readAsBinaryString(blob) method is called, the following steps must be followed:

1. If readyState = LOADING throw an InvalidStateError exception and terminate this algorithm.

2. Otherwise, initiate a read operation using the blob argument, and with the synchronous flag set. If the read operation returns failure, throw the appropriate exception as defined in §7.1 Throwing an Exception or Returning an Error. Terminate this algorithm.

3. If no error has occurred, return the result of the read operation as an binary string.

7. Errors and Exceptions

File read errors can occur when reading files from the underlying filesystem. The list below of potential error conditions is informative.

• The File or Blob being accessed may not exist at the time one of the asynchronous read methods or synchronous read methods are called. This may be due to it having been moved or deleted after a reference to it was acquired (e.g. concurrent modification with another application). See NotFoundError.

• A File or Blob may be unreadable. This may be due to permission problems that occur after a reference to a File or Blob has been acquired (e.g. concurrent lock with another application). Additionally, the snapshot state may have changed. See NotReadableError.

• User agents MAY determine that some files are unsafe for use within Web applications. A file may change on disk since the original file selection, thus resulting in an invalid read. Additionally, some file and directory structures may be considered restricted by the underlying filesystem; attempts to read from them may be considered a security violation. See §9 Security and Privacy Considerations and SecurityError.

7.1. Throwing an Exception or Returning an Error

This section is normative.

Error conditions can arise when reading a File or a Blob.

The read operation can terminate due to error conditions when reading a File or a Blob; the particular error condition that causes a read operation to return failure or queue a task to process read error is called a failure reason.

Synchronous read methods throw exceptions of the type in the table below if there has been an error owing to a particular failure reason.

Asynchronous read methods use the error attribute of the FileReader object, which must return a DOMException object of the most appropriate type from the table below if there has been an error owing to a particular failure reason, or otherwise return null.

8. A URL for Blob and File reference

This section defines a scheme for a URL used to refer to Blob objects (and File objects).

Note: other specifications, such as [MEDIA-SOURCE] extend this scheme to also refer to other types of objects.

8.1. Introduction

This section is informative.

Blob (or object) URLs are URLs like blob:http://example.com/550e8400-e29b-41d4-a716-446655440000. This enables integration of Blobs and Files with other Web Platform APIs that are only designed to be used with URLs, such as the img element. Blob URLs can also be used to navigate to as well as to trigger downloads of locally generated data.

For this purpose two static methods are exposed on the URL interface, createObjectURL(blob) and revokeObjectURL(url). The first method creates a mapping from a URL to a Blob, and the second method revokes said mapping. As long as the mapping exist the Blob can’t be garbage collected, so some care must be taken to revoke the URL as soon as the reference is no longer needed. All URLs are revoked when the global that created the URL itself goes away.

8.2. Model

Each user agent must maintain a blob URL store. A blob URL store is a map where keys are valid URL strings and values are blob URL Entries.

A blob URL entry consists of an object (typically a Blob, but other specs can extend this to refer to other types of objects), and an environment (an environment settings object).

Keys in the blob URL store (also known as blob URLs) are valid URL strings that when parsed result in a URL with a scheme equal to "blob", an empty host, and a path consisting of one element itself also a valid URL string.

8.2.1. Dereferencing Model for blob URLs

Futher requirements for the parsing an fetching model for blob URLs are defined in the [URL] and [Fetch] specifications.

8.2.2. Origin of blob URLs

Note: The effect of this algorithm is that the origin of a blob URL is always the same as that of the environment that created the URL, as long as the URL hasn’t been revoked yet. If the URL was revoked the serialization of the origin will still remain the same as the serialization of the origin of the environment that created the blob URL, but for opaque origins the origin itself might be distinct. This difference isn’t observable though, since a revoked blob URL can’t be resolved/fetched anymore anyway.

The [URL] spec should be updated to refer to this algorithm to resolve the origin of a blob URL when the URL is first parsed. This is tracked in issue #63 and in whatwg/url#127.

8.2.3. Lifetime of blob URLs

This specification extends the unloading document cleanup steps with the following steps:

1. Let environment be the Document's relevant settings object.

2. Let store be the user agent’s blob URL store;

3. Remove from store any entries for which the value's environment is equal to environment.

This needs a similar hook when a worker is unloaded.

8.3. Creating and Revoking a blob URL

Blob URLs are created and revoked using static methods exposed on the URL object. Revocation of a blob URL decouples the blob URL from the resource it refers to, and if it is dereferenced after it is revoked, user agents must act as if a network error has occurred. This section describes a supplemental interface to the URL specification [URL] and presents methods for blob URL creation and revocation.

[Exposed=(Window,DedicatedWorker,SharedWorker)]
partial interface URL {
  static DOMString createObjectURL(Blob blob);
  static void revokeObjectURL(DOMString url);
};

myurl = window1.URL.createObjectURL(myblob);
window2.URL.revokeObjectURL(myurl);

8.3.1. Examples of blob URL Creation and Revocation

Blob URLs are strings that are used to fetch Blob objects, and can persist for as long as the document from which they were minted using URL.createObjectURL()—see §8.2.3 Lifetime of blob URLs.

This section gives sample usage of creation and revocation of blob URLs with explanations.

url = URL.createObjectURL(blob);
img1.src = url;
img2.src = url;

var blobURLref = URL.createObjectURL(file);
img1 = new Image();
img2 = new Image();

// Both assignments below work as expected
img1.src = blobURLref;
img2.src = blobURLref;

// ... Following body load
// Check if both images have loaded
if(img1.complete && img2.complete) {
  // Ensure that subsequent refs throw an exception
  URL.revokeObjectURL(blobURLref);
} else {
  msg("Images cannot be previewed!");
  // revoke the string-based reference
  URL.revokeObjectURL(blobURLref);
}

The example above allows multiple references to a single blob URL, and the web developer then revokes the blob URL string after both image objects have been loaded. While not restricting number of uses of the blob URL offers more flexibility, it increases the likelihood of leaks; developers should pair it with a corresponding call to URL.revokeObjectURL().

9. Security and Privacy Considerations

This section is informative.

This specification allows web content to read files from the underlying file system, as well as provides a means for files to be accessed by unique identifiers, and as such is subject to some security considerations. This specification also assumes that the primary user interaction is with the <input type="file"/> element of HTML forms [HTML], and that all files that are being read by FileReader objects have first been selected by the user. Important security considerations include preventing malicious file selection attacks (selection looping), preventing access to system-sensitive files, and guarding against modifications of files on disk after a selection has taken place.

• Preventing selection looping. During file selection, a user may be bombarded with the file picker associated with <input type="file"/> (in a "must choose" loop that forces selection before the file picker is dismissed) and a user agent may prevent file access to any selections by making the FileList object returned be of size 0.

• System-sensitive files (e.g. files in /usr/bin, password files, and other native operating system executables) typically should not be exposed to web content, and should not be accessed via blob URLs. User agents may throw a SecurityError exception for synchronous read methods, or return a SecurityError exception for asynchronous reads.

This section is provisional; more security data may supplement this in subsequent drafts.

10. Requirements and Use Cases

This section covers what the requirements are for this API, as well as illustrates some use cases. This version of the API does not satisfy all use cases; subsequent versions may elect to address these.

• Once a user has given permission, user agents should provide the ability to read and parse data directly from a local file programmatically.

  A lyrics viewer. User wants to read song lyrics from songs in his plist file. User browses for plist file. File is opened, read, parsed, and presented to the user as a sortable, actionable list within a web application. User can select songs to fetch lyrics. User uses the "browse for file" dialog.

• Data should be able to be stored locally so that it is available for later use, which is useful for offline data access for web applications.

  A Calendar App. User’s company has a calendar. User wants to sync local events to company calendar, marked as "busy" slots (without leaking personal info). User browses for file and selects it. The text/calendar file is parsed in the browser, allowing the user to merge the files to one calendar view. The user wants to then save the file back to his local calendar file (using "Save As"?). The user can also send the integrated calendar file back to the server calendar store asynchronously.

• User agents should provide the ability to save a local file programmatically given an amount of data and a file name.

  Note: While this specification doesn’t provide an explicit API call to trigger downloads, the HTML5 specification has addressed this. The download attribute of the a element initiates a download, saving a File with the name specified. The combination of this API and the download attribute on a elements allows for the creation of files within web applications, and the ability to save them locally.

  A Spreadsheet App. User interacts with a form, and generates some input. The form then generates a CSV (Comma Separated Variables) output for the user to import into a spreadsheet, and uses "Save...". The generated output can also be directly integrated into a web-based spreadsheet, and uploaded asynchronously.

• User agents should provide a streamlined programmatic ability to send data from a file to a remote server that works more efficiently than form-based uploads today.

  A Video/Photo Upload App. User is able to select large files for upload, which can then be "chunk-transfered" to the server.

• User agents should provide an API exposed to script that exposes the features above. The user is notified by UI anytime interaction with the file system takes place, giving the user full ability to cancel or abort the transaction. The user is notified of any file selections, and can cancel these. No invocations to these APIs occur silently without user intervention.

Acknowledgements

This specification was originally developed by the SVG Working Group. Many thanks to Mark Baker and Anne van Kesteren for their feedback.

Thanks to Robin Berjon and Jonas Sicking for editing the original specification.

Special thanks to Olli Pettay, Nikunj Mehta, Garrett Smith, Aaron Boodman, Michael Nordman, Jian Li, Dmitry Titov, Ian Hickson, Darin Fisher, Sam Weinig, Adrian Bateman and Julian Reschke.

Thanks to the W3C WebApps WG, and to participants on the public-webapps@w3.org listserv