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
[Exposed =(Window ,Worker ),Serializable ]interface {
Blob constructor (optional sequence <BlobPart >,
blobParts optional BlobPropertyBag = {});
options Blobreadonly attribute unsigned long long size ;readonly attribute DOMString type ;object refers to a byte sequence, and has a// slice Blob into byte-ranged chunksBlob slice (optional [Clamp ]long long ,
start optional [Clamp ]long long ,
end optional DOMString );
contentType size// read from the Blob. [NewObject ]ReadableStream stream (); [NewObject ]Promise <USVString >text (); [NewObject ]Promise <ArrayBuffer >arrayBuffer (); };attribute which is the total number of bytes in the byte sequence, and aenum {
EndingType ,
"transparent" };
"native" dictionary {
BlobPropertyBag DOMString = "";
type EndingType = "transparent"; };
endings typedef (BufferSource or Blob or USVString );
BlobPart
A
type
Blob
attribute,
which
is
has
an
associated
[[type]]
internal
slot,
an
ASCII-encoded
string
in
lower
case
representing
the
media
type
of
the
byte
sequence.
Each
A
Blob
must
have
has
an
associated
[[data]]
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
File
s.
slot,
a
blob
data
description
.
Blob
objects
are
serializable
objects
.
Their
serialization
steps
-
If forStorage is true:
Let bytes be the result of reading all bytes from value .
Set serialized .[[BlobData]] to the result of creating blob data from bytes given bytes .
In at least Chrome’s IndexedDB implementation, this copying of the data of blobs is only done when a transaction is committed (and failure to read the blob will cause the commit to fail).
Otherwise:
-
Set serialized
.[[SnapshotState]].[[BlobData]] to value’s snapshot state. [[data]] .
-
-
Set serialized
.[[ByteSequence]].[[Type]] to value’s underlying byte sequence.. [[type]] .
The actual storage API serialized was persisted in will need a way of modifying the read algorithms for deserialized blobs. I.e. a Blob that was deserialized from IndexedDB should start throwing in its read steps after clear-site-data clears all IndexedDB data. Somehow let StructuredDeserialize pass along a hook from the storage API to here? <https://github.com/w3c/webappsec-clear-site-data/issues/49>
Blob
blob
has
an
associated
get
stream
algorithm,
which
runs
these
steps:
-
Let stream be the result of constructing a
ReadableStream
object. -
Run the following steps in parallel :
-
While not all bytes ofLet blob datahave been read:be blob . [[data]] . -
Let
bytesread state be thebyte sequenceresult of calling blob data ’s read initialization algorithm given blob data ’s snapshot state and 0. If thatresults from reading a chunkthrew an exception, errorfromstream with that exception and abort these steps. While true:
Let read result be the result of calling blob data ’s read algorithm given read state .
-
If
athat threw an exception (a file read erroroccured while reading bytes ,), error stream withathat failure reason and abort these steps. -
If read result is end of blob , break .
Enqueue a
Uint8Array
object wrapping anArrayBuffer
containingbytesread result into stream . If that threw an exception, error stream with that exception and abort these steps.
We need to specify more concretely what reading from a Blob actually does, what possible errors can happen, perhaps something about chunk sizes, etc. <https://github.com/w3c/FileAPI/issues/144>
-
-
Return stream .
3.1.
Constructors
Concepts
If
invoked
with
zero
parameters,
return
The
data
represented
by
a
new
Blob
object
is
described
by
a
blob
data
description
,
consisting
of
0
bytes,
some
representation
of
the
data,
combined
with
size
a
set
of
algorithms
to
0,
return
the
actual
data
as
a
series
of
byte
sequences
.
A blob data description has an associated size , a number specifying the total number of bytes in the byte sequence represented by the blob.
A blob data description has an associated snapshot state . This is a map that represents the data stored in the blob.
A
blob
data
description
has
an
associated
read
initialization
algorithm
.
This
algorithm
takes
two
arguments:
the
snapshot
state,
and
with
a
byte
offset.
It
returns
a
struct
which
will
be
used
as
input
for
the
read
algorithm
.
A blob data description has an associated read algorithm . This algorithm takes one argument (the struct returned by the read initialization algorithm ). It returns either a byte sequence or the special end of blob value.
type
Blob
Let bytes be an empty
string.byte sequence .-
Let
bytesblob data be blob . [[data]] . Let read state be the result of
processingcalling blobpartsdata ’s read initialization algorithm givenblobPartsblob data ’s snapshot state andoptions .0.-
If the type member of the options argument is not the empty string, run the following sub-steps:While true:-
Let
tread result be thetype dictionary member. Ifresult of callingtblob datacontains any characters outside the range U+0020 to U+007E, then set’s read algorithm giventread state . If read result
to the empty string and return from these substeps.is end of blob , break .-
Convert every character inAppendtread result toASCII lowercase .bytes .
-
-
Return bytes .
Conceptually
a
Blob
object
referring
to
bytes
as
its
associated
byte
sequence,
with
its
represents
a
snapshot
of
some
amount
of
data,
and
is
frozen
in
time
at
the
time
a
size
Blob
set
to
the
length
instance
is
created.
As
such
for
any
specific
instance
of
bytes
,
and
its
a
,
every
invocation
of
read
all
bytes
should
either
return
the
exact
same
byte
sequence
,
or
throw
an
exception.
Additionally
the
returned
byte
sequence
's
length
should
be
equal
to
the
type
Blob
set
value
of
t
from
blob
[[data]]
's
size
.
Note:
This
is
a
non-trivial
requirement
to
implement
for
user
agents,
especially
when
a
blob
is
backed
by
a
file
on
disk
(i.e.
was
created
by
the
substeps
above.
create
a
file
backed
File
object
algorithm).
User
agents
can
use
modification
time
stamps
and
other
mechanisms
to
maintain
this
requirement,
but
this
is
left
as
an
implementation
detail.
3.1.1.
Constructor
Parameters
Byte
Sequence
backed
blobs
The
Blob()
snapshot
state
constructor
can
be
invoked
with
the
parameters
below:
A
for
a
byte
sequence
backed
blob
contains
a
member,
a
byte
sequence
blobParts
"data"
which
takes
any
number
of
.
-
Let blob data be a new blob data description .
Set blob data . snapshot state [
BufferSource"data"elements.] to bytes .-
BlobSet blob data . read initialization algorithmelements.to the bytes blob read initialization steps . -
USVStringSet blob data . read algorithmelements.to the bytes blob read steps . -
Return blob data .
A
bytes
blob
read
state
is
a
struct
An
optional
BlobPropertyBag
conssting
of
bytes
(a
byte
sequence
which
takes
these
optional
members:
).
-
Let read state be a new bytes blob read state .
If offset is larger than snapshot state [
type, the ASCII-encoded string in lower case representing the media type of the Blob"data". Normative conditions for this member are provided in the § 3.1 Constructors-
Otherwise, set read state . bytes to a copy of snapshot state [
"data"
] with the first offset bytes removed.endings Return read state .
Let result be read state . bytes .
Set read state . bytes to an
enum which can take the valuesempty byte sequence .If result is empty, return end of blob .
Return result .
"transparent"
3.1.2.
or
Multipart
blobs
"native"
.
By
default
this
is
set
to
Blobs
created
by
the
and
"transparent"
Blob
.
If
set
to
constructors
are
made
up
of
multiple
parts,
where
each
part
could
be
"native"
File
,
line
endings
will
converted
to
native
in
any
USVString
a
blob
itself.
The
snapshot
state
for
a
multipart
blob
contains
a
"parts"
elements
in
blobParts
member,
a
list
.
of
blob
data
descriptions
.
BlobPart
's
BlobPropertyBag
options
,
run
the
following
steps:
-
Let size be 0.
Let parts be an empty list .
Let bytes be an empty byte sequence
of bytes..-
For each element in
partsblobParts :-
If element is a
USVString
, run the following substeps:-
Let s be element .
-
If the
endings
member of options is"native"
, set s to the result of converting line endings to native of element . -
Append the result of UTF-8 encoding s to bytes .
Note: The algorithm from WebIDL [WebIDL] replaces unmatched surrogates in an invalid utf-16 string with U+FFFD replacement characters. Scenarios exist when the
Blob
constructor may result in some data loss due to lost or scrambled character sequences.
-
-
If element is a
BufferSource
, get a copy of the bytes held by the buffer source , and append those bytes to bytes . -
If element is a
Blob
:, append-
If bytes is not empty:
Let part be the result of creating blob data from bytes
it representsgiven bytes .Append part to parts .
Set size to size + part . size .
Set bytes to an empty byte sequence .
Let part be element . [[data]] .
Append part to parts .
Set size to size + part . size .
Note: The
type
of theBlob
array element is ignored and will not affecttype
of returnedBlob
object. -
-
-
If bytes is not empty:
Let part be the result of creating blob data from bytes given bytes .
Append part to parts .
Set size to size + part . size .
Let result be a blob data description .
Set result . size to size .
Set result . snapshot state [
"parts"
] to parts .Set result . read initialization algorithm to the multipart blob read initialization steps .
Set result . read algorithm to the multipart blob read steps .
Return result .
A multipart blob read state is a struct consisting of:
- parts
A queue of blob data descriptions , representing the not yet read parts of the blob.
- offset
A number, representing the byte offset in the remaining blob parts from which to start returning data.
- nested blob data
undefined
or a blob data description . This isundefined
unless otherwise specified.- nested read state
undefined
or a struct , representing the read state for a nested read operation. This isundefined
unless otherwise specified.
Let read state be a new multipart blob read state .
Set read state . parts to a clone of snapshot state [
"parts"
].Set read state . offset to offset .
Return read state .
Let result be end of blob .
While result is end of blob :
If read state . nested read state is not
undefined
:Set result to the result of calling read state . nested blob data 's read algorithm given read state . nested read state .
If result is end of blob :
Set read state . nested read state to
undefined
.Set read state . nested blob data to
undefined
.
Otherwise:
If read state . parts is empty:
Return end of blob .
Let current part be the result of dequeueing from read state . parts .
Set read state . nested blob data to current part .
Set read state . nested read state to the result of calling current part ’s read initialization algorithm given current part ’s snapshot state and read state . offset .
Set read state . offset to 0.
Return result .
3.1.3. Sliced blobs
Blobs
created
by
the
slice()
method
are
also
known
as
sliced
blobs.
The
snapshot
state
for
a
sliced
blob
contains
a
"offset"
member
(a
number),
a
"span"
member
(a
number),
and
a
"source"
member
(a
blob
data
description
).
A sliced blob read state is a struct consisting of:
- source
A blob data description , representing the blob that was sliced.
- bytes remaining
A number, representing the remaining number of bytes to be returned.
- nested read state
A struct , representing the read state of the nested read operation.
Let read state be a new sliced blob read state .
Set read state . bytes remaining to snapshot state [
"span"
] - offset .Let read offset be snapshot state [
"offset"
] + offset .Set read state . nested read state to the result of calling read state . source 's read initialization algorithm given read state . source 's snapshot state and read offset .
Return read state .
If read state . bytes remaining <= 0:
Return end of blob .
Let result be the result of calling read state . source 's read algorithm given read state . nested read state .
If result is not end of blob :
If result ’s length is larger than read state . bytes remaining :
Truncate result to be read state . bytes remaining bytes long.
Set read state . bytes remaining to read state . bytes remaining - result ’s length .
Return result .
3.2. Constructors
new
Blob(
blobParts
,
options
)
constructor
steps
are:
Let blob data be the result of processing blob parts given blobParts and options .
Let type be an empty string.
If the
type
member of theoptions
argument is not the empty string, run the following sub-steps:Let type be the
type
dictionary member. If type contains any characters outside the range U+0020 to U+007E, then set type to the empty string and return from these substeps.Convert every character in type to ASCII lowercase .
-
Let native line ending be be the code point U+000A LF.
-
If the underlying platform’s conventions are to represent newlines as a carriage return and line feed sequence, set native line ending to the code point U+000D CR followed by the code point U+000A LF.
-
Set result to the empty string .
-
Let position be a position variable for s , initially pointing at the start of s .
-
Let token be the result of collecting a sequence of code points that are not equal to U+000A LF or U+000D CR from s given position .
-
Append token to result .
-
While position is not past the end of s :
-
If the code point at position within s equals U+000D CR:
-
Append native line ending to result .
-
Advance position by 1.
-
If position is not past the end of s and the code point at position within s equals U+000A LF advance position by 1.
-
-
Otherwise if the code point at position within s equals U+000A LF, advance position by 1 and append native line ending to result .
-
Let token be the result of collecting a sequence of code points that are not equal to U+000A LF or U+000D CR from s given position .
-
Append token to result .
-
-
Return result .
// 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.
3.3.
Attributes
-
blob
.
size
, of type unsigned long long , readonly -
Returns the size of the byte sequence represented by blob in number of bytes.
On getting, conforming user agents must return the total number of bytes that can be read by a FileReader
The
size
or
FileReaderSync
getter
steps
are
to
return
this
.
[[data]]
.
size
.
-
object, or 0 if theblob .Blobtypehas 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 aBlobas an ASCII-encoded string in lower case, such that when it is converted to a byte sequence, it is a parsable MIME type ,thean 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 theslice()
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
tof aBlob
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 failure.Note: Use of the
type
attribute informs the package data algorithm and determines theContent-Type
header when fetching blob URLs .
The
type
getter
steps
are
to
return
this
.
[[type]]
.
3.3.
3.4.
Methods
and
Parameters
3.3.1.
3.4.1.
The
slice()
method
-
slice
=
blob
.
slice
( start , end , contentType )slice() method returnsslice is a newBlob
objectobject, sharing storage with blob , with bytes ranging from the optional start parameter up to but not including the optional end parameter, and with atype
attribute that is the value of the optional contentType parameter.It must act as follows:LetNegative values forOstartbeand end are interpreted as relative to theBlob context object on whichend of theslice() method is being called.blob .
slice(
start
,
end
,
contentType
)
Let relativeStart be 0.
-
If start is
negative, letnotundefined
: -
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: If the optional end parameter is not used as a parameter when making this call, letLet relativeEnd be this . [[data]] . size.. -
If end is
negative, letnotundefined
: -
The optionalLet span becontentType 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:max((relativeEnd - relativeStart), 0)If the contentType parameter is not provided, let -
Let relativeContentType be
set to thean empty string.Else let -
If
relativeContentType be set tocontentTypeand run the substeps below:is notundefined
:-
If
relativeContentTypecontentTypecontainsdoes not contain any characters outside the range ofU+0020 to U+007E, then set relativeContentTypeU+0x0020 tothe empty string and return from these substeps.U+0x007E:-
Convert every character inSet relativeContentType to ASCII lowercase.of contentType .
-
-
-
Let
spansnapshot state bemax((relativeEnd - relativeStart), 0) .a new map . -
Return a newSet snapshot state [Blob"offset"object] toS with the following characteristics:relativeStart . -
Set
Ssnapshot staterefers["span"
] to span . -
Set snapshot state
consecutive byte["source"
] to this . [[data]] .s from Let
O , beginning with the byteresult be a newBlob
object.at byte-order positionSet
relativeStartresult . [[type]] toSrelativeContentType .-
Set
Sresult .type[[data]] . snapshot state=torelativeContentTypesnapshot state . -
Set result . [[data]] . read initialization algorithm to the sliced blob read initialization steps .
Set result . [[data]] . read algorithm to the sliced blob read steps .
Return result .
slice()
calls
possible.
Since
the
File
interface
inherits
from
the
Blob
interface,
examples
are
based
on
the
use
of
the
File
interface.
// 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" ); }
3.3.2.
3.4.2.
The
stream()
method
The
stream()
method,
when
invoked,
must
return
the
result
of
calling
get
stream
on
the
context
object
.
3.3.3.
3.4.3.
The
text()
method
The
text()
method,
when
invoked,
must
run
these
steps:
-
Let
streampromise bethe result of calling get stream on the context objecta new Promise . -
Run the following steps in parallel :
Let
readerbytes be the result ofgetting a readerreading all bytes fromstream .this . If that threw an exception,return a newreject promiserejectedwith thatexception.exception and abort.-
LetResolve promisebewith the result ofreading all bytesrunning UTF-8 decodefrom stream withonreaderbytes .
-
Return
the result of transformingpromiseby a fulfillment handler that returns the result of running UTF-8 decode on its first argument..
Note:
This
is
different
from
the
behavior
of
readAsText()
to
align
better
with
the
behavior
of
Fetch’s
text()
.
Specifically
this
method
will
always
use
UTF-8
as
encoding,
while
FileReader
can
use
a
different
encoding
depending
on
the
blob’s
type
and
passed
in
encoding
name.
3.3.4.
3.4.4.
The
arrayBuffer()
method
The
arrayBuffer()
method,
when
invoked,
must
run
these
steps:
-
Let
streampromise bethe result of calling get stream on the context objecta new Promise . -
Let reader beRun theresult of getting a reader from stream . If that threw an exception, return a new promise rejected with that exception.following steps in parallel :-
Let
promisebytes be the result of reading all bytes from this . If that threw an exception, rejectstreampromise withreader .that exception and abort. -
Return the result of transformingResolve promiseby a fulfillment handler that returnswith a newArrayBuffer
whose contents areits first argument.bytes .
-
Return promise .
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.
[Exposed =(Window ,Worker ),Serializable ]interface :
File Blob {constructor (sequence <BlobPart >,
fileBits USVString ,
fileName optional FilePropertyBag = {});
options readonly attribute DOMString name ;readonly attribute long long lastModified ; };dictionary :
FilePropertyBag BlobPropertyBag {long long ; };
lastModified
If
a
A
File
object
is
a
reference
to
a
byte
sequence
originating
from
has
an
associated
[[name]]
intenral
slot,
a
file
on
disk,
then
its
snapshot
state
should
be
set
to
the
state
of
the
file
on
disk
at
the
time
the
string.
A
File
object
is
created.
has
an
associated
[[lastModified]]
internal
slot,
something.
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
objects
are
serializable
objects
.
Their
serialization
steps
,
given
value
,
serialized
and
forStorage
,
are:
Invoke the blob serialization steps
setgiven value , serialized and forStorage .Set serialized .[[Name]] to value . [[name]] .
Set serialized .[[LastModified]] to value . [[lastModified]] .
Their deserialization steps , given value and serialized , are:
Invoke the blob deserialization steps given value and serialized .
Set value . [[name]] to serialized .[[Name]].
Set value . [[lastModified]] to serialized .[[LastModified]].
4.1. Concepts
The
snapshot
state
of
the
underlying
storage
on
disk
at
the
time
the
for
a
file
backed
blob
contains
a
"file"
member
(a
reference
is
taken.
If
the
to
a
native
file
is
modified
on
disk
following
the
time
disk),
and
a
reference
has
been
taken,
the
"last
modified"
member
(a
number).
File
Let snapshot state be an empty map .
Set snapshot state [
"file"
] to native file .will differ fromLet last modified be the
statelast time native file was modified, as the number of milliseconds since theunderlying storage. User agents may use modification time stamps and other mechanisms to maintain snapshot state , butUnix Epoch . If thisis leftcan’t be determined, set last modified to the current date and time represented asan implementation detail.the number of milliseconds since the Unix Epoch .-
When aSet snapshot state [File"last modified"object refers] toalast modified . Let name be the file
on disk,name of native file , converted to a string in a useragents must returnagent defined manner.Let content type be the mime type of
that file, and must follownative file (as a lowercase ASCII string), derived from name in a user agent defined manner, or an empty string if no type could be determined, taking into account the following file type guidelinesbelow::-
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.
] { , , = {}); ; ; }; { ; };-
-
Let result be a new
File
objects are serializable objects . Their serialization steps , givenobject. Set
value andresult . [[type]] toserialized , are:content type .-
Set
serialized .[[SnapshotState]]result . [[data]] . size to the size of native file . Set result . [[data]] . snapshot state to
value ’ssnapshot state .-
Set result . [[data]] . read initialization algorithm to the file read initialization steps .
-
Set
serialized .[[ByteSequence]]result . [[data]] . read algorithm to the file read steps . Set
value ’s underlying byte sequence.result . [[name]] to name .-
Set
serialized .[[Name]]result . [[lastModified]] tothe valuelast modified . Return result .
A file blob read state is a struct consisting of a file handle (a not further defined handle to a file that is open for reading), and a offset (a number).
Let file be snapshot state [
name"file"attribute.].-
Set serialized .[[LastModified]] toIf thevalue offile referred tovaluefile’sno longer exists, throw alastModifiedNotFoundErrorattribute. -
Their deserialization steps , givenLetvaluelast modifiedandbe the last timeserialized , are:file was modified, as the number of milliseconds since the Unix Epoch . -
SetIfvaluelast modified’sis different from snapshot state ["last modified"
], throw aNotReadableError
. User agents may attempt to
serialized .[[SnapshotState]].detect in other ways that the file on disk has been changed. If this is the case, throw aNotReadableError
.-
SetIf the user agent for some other reason decides that the file should not be read by a website, throw aSecurityError
. Let
valueread state’s underlying byte sequence to serialized .[[ByteSequence]].be a new file blob read state .-
InitializeSet read state . file handle to thevalueresult of openingvaluefile’sfor reading. If this fails, for example due to permission problems, throw anameNotReadableErrorattribute -
Set read state . offset to
serialized .[[Name]].offset . -
InitializeReturn read state .
Let bytes be the
valueresult of reading bytes fromvalue ’sread state . file handle at read state . offset .If reading bytes failed other than by reaching the end of the file, throw a
lastModifiedNotReadableErrorattribute-
If reading bytes failed becaus the end of the file was reached, return end of blob .
Set read state . offset to
serialized .[[LastModified]].read state . offset + bytes ’s length .Return bytes .
4.1.
4.2.
Constructor
new
File(
fileBits
,
fileName
,
options
)
-
Let bytes beRun theresult of processing blob parts givenfileBitsBloband( fileBits , options.) constructor steps. -
Let n be a new string of the same size as
thefileNameargument to the constructor.. -
Copy every character from fileName to n , replacing any "/" character (U+002F SOLIDUS) with a ":" (U+003A COLON).
Note: Underlying OS filesystems use differing conventions for file name; with constructed files, mandating UTF-16 lessens ambiquity when file names are converted to byte
sequences.sequences . -
ProcessIf options .FilePropertyBaglastModifieddictionary argument by running the following substeps:member is provided:-
If the type member is provided and is not the empty string, letLettd beset to theoptions .typelastModifiedIf t contains any characters outside the range U+0020 to U+007E, then set t to the empty string and return from these substeps.
-
-
Convert every character in t to ASCII lowercase .Otherwise:-
If the lastModified member is provided, letLet d beset to the lastModified dictionary member. If it is not provided, set d tothe current date and time represented as the number of milliseconds since the Unix Epoch (which is the equivalent ofDate.now()
[ECMA-262] ).Note: Since ECMA-262
Date
objects convert tolong long
values representing the number of milliseconds since the Unix Epoch , thelastModified
member could be aDate
object [ECMA-262] .
-
-
Return a new File object F such that: F refers to the bytes byte sequence. F . size is set to the number of total bytes in bytes . F . nameSet this . [[name]]is setto n . -
F . type is set to t . F . lastModifiedSet this . [[lastModified]]is setto d .
4.1.1.
4.3.
Constructor
Parameters
Attributes
The
File()
constructor
can
be
invoked
with
the
parameters
below:
-
parameter Afile .USVString parameter representing thenameof 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 BlobPropertyBagtakes 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 returnReturns 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, theymustreturn the empty string.If a File
The
name
object
is
created
using
a
constructor,
further
normative
conditions
for
this
attribute
getter
steps
are
found
in
§ 4.1
Constructor
to
return
this
.
[[name]]
.
-
file
.
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 longset to -
Return a 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 returnreturns the current date and timeas a long long representing the number of milliseconds since the Unix Epoch ; this is equivalent(equivalent toDate
[ECMA-262]. now() . If a Fileobject is created using a constructor, further normative conditions for this attribute are found in § 4.1 Constructor .).
The
File
lastModified
interface
is
available
on
objects
that
expose
an
attribute
of
type
FileList
;
these
objects
getter
steps
are
defined
in
HTML
[HTML]
to
return
this
.
[[lastModified]]
.
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
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:
-
Set serialized .[[Files]] to an empty list .
-
For each file in value , append the sub-serialization of file to serialized .[[Files]].
Their deserialization step , given serialized and value , are:
-
For each file of serialized .[[Files]], add the sub-deserialization of file to value .
<input
type="file">
element
within
a
form,
and
then
accessing
selected
files.
// 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
index
th
File
object in theFileList
. If there is no index thFile
object in theFileList
, then this method must returnnull
.index
must be treated by user agents as value for the position of aFile
object in theFileList
, with 0 representing the first file. Supported property indices are the numbers in the range zero to one less than the number ofFile
objects represented by theFileList
object. If there are no suchFile
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 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.
6.2.
The
FileReader
API
[Exposed =(Window ,Worker )]interface :
FileReader EventTarget {constructor (); // async read methods); ); ); );void readAsArrayBuffer (Blob );
blob void readAsBinaryString (Blob );
blob void readAsText (Blob ,
blob optional DOMString );
encoding void readAsDataURL (Blob );
blob void abort (); // statesconst unsigned short = 0;
EMPTY const unsigned short = 1;
LOADING const unsigned short = 2;
DONE readonly attribute unsigned short readyState ; // File or Blob data;readonly attribute (DOMString or ArrayBuffer )?result ;;readonly attribute DOMException ?error ; // event handler content attributesattribute EventHandler onloadstart ;attribute EventHandler onprogress ;attribute EventHandler onload ;attribute EventHandler onabort ;attribute EventHandler onerror ;attribute EventHandler onloadend ; };
A
FileReader
has
an
associated
state
,
that
is
"empty"
,
"loading"
,
or
"done"
.
It
is
initially
"empty"
.
A
FileReader
has
an
associated
result
(
null
,
a
DOMString
or
an
ArrayBuffer
).
It
is
initially
null
.
A
FileReader
has
an
associated
error
(
null
or
a
DOMException
).
It
is
initially
null
.
The
FileReader()
constructor,
when
invoked,
must
return
a
new
FileReader
object.
The
readyState
attribute’s
getter,
when
invoked,
switches
on
the
context
object
's
state
and
runs
the
associated
step:
The
result
attribute’s
getter,
when
invoked,
must
return
the
context
object
's
result
.
The
error
attribute’s
getter,
when
invoked,
must
return
the
context
object
's
error
.
FileReader
fr
has
an
associated
read
operation
algorithm,
which
given
blob
,
a
type
and
an
optional
encodingName
,
runs
the
following
steps:
-
If fr ’s state is
"loading"
, throw anInvalidStateError
DOMException
. -
Set fr ’s state to
"loading"
. -
Set fr ’s result to
null
. -
Set fr ’s error to
null
. -
Let stream be the result of calling get stream on blob .
-
Let reader be the result of getting a reader from stream .
-
Let bytes by an empty byte sequence .
-
Let chunkPromise be the result of reading a chunk from stream with reader .
-
Let isFirstChunk be true.
-
In parallel , while true:
-
Wait for chunkPromise to be fulfilled or rejected.
-
If chunkPromise is fulfilled, and isFirstChunk is true, queue a task to fire a progress event called
loadstart
at fr .We might change
loadstart
to be dispatched synchronously, to align with XMLHttpRequest behavior. <https://github.com/w3c/FileAPI/issues/119> -
Set isFirstChunk to false.
-
If chunkPromise is fulfilled with an object whose
done
property is false and whosevalue
property is aUint8Array
object, run these steps:-
Let bs be the byte sequence represented by the
Uint8Array
object. -
Append bs to bytes .
-
If roughly 50ms have passed since these steps were last invoked, queue a task to fire a progress event called
progress
at fr . -
Set chunkPromise to the result of reading a chunk from stream with reader .
-
-
Otherwise, if chunkPromise is fulfilled with an object whose
done
property is true, queue a task to run the following steps and abort this algorithm:-
Set fr ’s state to
"done"
. -
Let result be the result of package data given bytes , type , blob ’s
type
, and encodingName . -
If package data threw an exception error :
-
Set fr ’s error to error .
-
Fire a progress event called
error
at fr .
-
-
Else:
-
Set fr ’s result to result .
-
Fire a progress event called
load
at the fr .
-
-
If fr ’s state is not
"loading"
, fire a progress event calledloadend
at the fr .Note: Event handler for the
load
orerror
events could have started another load, if that happens theloadend
event for this load is not fired.
-
-
Otherwise, if chunkPromise is rejected with an error error , queue a task to run the following steps and abort this algorithm:
-
Set fr ’s state to
"done"
. -
Set fr ’s error to error .
-
Fire a progress event called
error
at fr . -
If fr ’s state is not
"loading"
, fire a progress event calledloadend
at fr .Note: Event handler for the
error
event could have started another load, if that happens theloadend
event for this load is not fired.
-
-
Use the file reading task source for all these tasks.
6.2.1. 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:
event handler content attribute | event handler event type |
---|---|
onloadstart
|
loadstart
|
onprogress
|
progress
|
onabort
|
abort
|
onerror
|
error
|
onload
|
load
|
onloadend
|
loadend
|
6.2.2. FileReader States
FileReader
object
can
be
in
one
of
3
states.
The
readyState
attribute
tells
you
in
which
state
the
object
is:
-
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 mintedFileReader
object, until one of the read methods have been called on it. -
LOADING
(numeric value 1) -
A
File
orBlob
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
orBlob
has been read into memory, OR a file read error occurred, OR the read was aborted usingabort()
. TheFileReader
is no longer reading aFile
orBlob
. IfreadyState
is set toDONE
it means at least one of the read methods have been called on thisFileReader
.
6.2.3. Reading a File or Blob
The
FileReader
interface
makes
available
several
asynchronous
read
methods
—
readAsArrayBuffer()
,
readAsBinaryString()
,
readAsText()
and
readAsDataURL()
,
which
read
files
into
memory.
Note:
If
multiple
concurrent
read
methods
are
called
on
the
same
FileReader
object,
user
agents
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
.)
6.2.3.1.
The
readAsDataURL()
method
The
readAsDataURL(
blob
)
method,
when
invoked,
must
initiate
a
read
operation
for
blob
with
DataURL
.
6.2.3.2.
The
readAsText()
method
The
readAsText(
blob
,
encoding
)
method,
when
invoked,
must
initiate
a
read
operation
for
blob
with
Text
and
encoding
.
6.2.3.3.
The
readAsArrayBuffer()
The
readAsArrayBuffer(
blob
)
method,
when
invoked,
must
initiate
a
read
operation
for
blob
with
ArrayBuffer
.
6.2.3.4.
The
readAsBinaryString()
method
The
readAsBinaryString(
blob
)
method,
when
invoked,
must
initiate
a
read
operation
for
blob
with
BinaryString
.
Note:
The
use
of
readAsArrayBuffer()
is
preferred
over
readAsBinaryString()
,
which
is
provided
for
backwards
compatibility.
6.2.3.5.
The
abort()
method
When
the
abort()
method
is
called,
the
user
agent
must
run
the
steps
below:
-
If context object 's state is
"empty"
or if context object 's state is"done"
set context object 's result tonull
and terminate this algorithm . -
If context object 's state is
"loading"
set context object 's state to"done"
and set context object 's result tonull
. -
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.
-
Terminate the algorithm for the read method being processed.
-
Fire a progress event called
abort
at the context object . -
If context object 's state is not
"loading"
, fire a progress event calledloadend
at the context object .
6.3. Packaging data
Blob
has
an
associated
package
data
algorithm,
given
bytes
,
a
type
,
a
optional
mimeType
,
and
a
optional
encodingName
,
which
switches
on
type
and
runs
the
associated
steps:
- DataURL
-
Return bytes as a DataURL [RFC2397] subject to the considerations below:
-
Use mimeType as part of the Data URL if it is available in keeping with the Data URL specification [RFC2397] .
-
If mimeType is not available return a Data URL without a media-type. [RFC2397] .
Better specify how the DataURL is generated. <https://github.com/w3c/FileAPI/issues/104>
-
- Text
-
-
Let encoding be failure.
-
If the encodingName is present, set encoding to the result of getting an encoding from encodingName .
-
If encoding is failure, and mimeType is present:
-
Let type be the result of parse a MIME type given mimeType .
-
If type is not failure, set encoding to the result of getting an encoding from type ’s parameters [
"charset"
].Ifblob
has atype
attribute oftext/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.
-
-
If encoding is failure, then set encoding to UTF-8 .
-
Decode bytes using fallback encoding encoding , and return the result.
-
- ArrayBuffer
-
Return a new
ArrayBuffer
whose contents are bytes . - BinaryString
-
Return bytes as a binary string, in which every byte is represented by a code unit of equal value [0..255].
6.4. 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]
6.4.1. Event Summary
The
following
are
the
events
that
are
fired
at
FileReader
objects.
Event name | Interface | Fired when… |
---|---|---|
loadstart
|
ProgressEvent
| When the read starts. |
progress
|
ProgressEvent
|
While
reading
(and
decoding)
blob
|
abort
|
ProgressEvent
|
When
the
read
has
been
aborted.
For
instance,
by
invoking
the
abort()
method.
|
error
|
ProgressEvent
| When the read has failed (see file read errors ). |
load
|
ProgressEvent
| When the read has successfully completed. |
loadend
|
ProgressEvent
| When the request has completed (either in success or failure). |
6.4.2. 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:
-
Once a
loadstart
has been fired, a correspondingloadend
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
andloadend
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 -
-
One
progress
event will fire whenblob
has been completely read into memory. -
No
progress
event fires after any one ofabort
,load
, anderror
have fired. At most one ofabort
,load
, anderror
fire for a given read.
6.5. 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).
6.5.1.
The
FileReaderSync
API
This
interface
provides
methods
to
synchronously
read
File
or
Blob
objects
into
memory.
[Exposed =(DedicatedWorker ,SharedWorker )]interface {
FileReaderSync (); // Synchronously return strings
constructor ); ); ); );ArrayBuffer readAsArrayBuffer (Blob );
blob DOMString readAsBinaryString (Blob );
blob DOMString readAsText (Blob ,
blob optional DOMString );
encoding DOMString readAsDataURL (Blob ); };
blob
6.5.1.1. Constructors
When
the
FileReaderSync()
constructor
is
invoked,
the
user
agent
must
return
a
new
FileReaderSync
object.
6.5.1.2.
The
readAsText()
The
readAsText(
blob
,
encoding
)
method,
when
invoked,
must
run
these
steps:
-
Let stream be the result of calling get stream on blob .
-
Let reader be the result of getting a reader from stream .
-
Let promise be the result of reading all bytes from stream with reader .
-
Wait for promise to be fulfilled or rejected.
-
If promise fulfilled with a byte sequence bytes :
-
Return the result of package data given bytes , Text , blob ’s
type
, and encoding .
-
-
Throw promise ’s rejection reason.
6.5.1.3.
The
readAsDataURL()
method
The
readAsDataURL(
blob
)
method,
when
invoked,
must
run
these
steps:
-
Let stream be the result of calling get stream on blob .
-
Let reader be the result of getting a reader from stream .
-
Let promise be the result of reading all bytes from stream with reader .
-
Wait for promise to be fulfilled or rejected.
-
If promise fulfilled with a byte sequence bytes :
-
Return the result of package data given bytes , DataURL , and blob ’s
type
.
-
-
Throw promise ’s rejection reason.
6.5.1.4.
The
readAsArrayBuffer()
method
The
readAsArrayBuffer(
blob
)
method,
when
invoked,
must
run
these
steps:
-
Let stream be the result of calling get stream on blob .
-
Let reader be the result of getting a reader from stream .
-
Let promise be the result of reading all bytes from stream with reader .
-
Wait for promise to be fulfilled or rejected.
-
If promise fulfilled with a byte sequence bytes :
-
Return the result of package data given bytes , ArrayBuffer , and blob ’s
type
.
-
-
Throw promise ’s rejection reason.
6.5.1.5.
The
readAsBinaryString()
method
The
readAsBinaryString(
blob
)
method,
when
invoked,
must
run
these
steps:
-
Let stream be the result of calling get stream on blob .
-
Let reader be the result of getting a reader from stream .
-
Let promise be the result of reading all bytes from stream with reader .
-
Wait for promise to be fulfilled or rejected.
-
If promise fulfilled with a byte sequence bytes :
-
Return the result of package data given bytes , BinaryString , and blob ’s
type
.
-
-
Throw promise ’s rejection reason.
Note:
The
use
of
readAsArrayBuffer()
is
preferred
over
readAsBinaryString()
,
which
is
provided
for
backwards
compatibility.
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
orBlob
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). SeeNotFoundError
. -
A
File
orBlob
may be unreadable. This may be due to permission problems that occur after a reference to aFile
orBlob
has been acquired (e.g. concurrent lock with another application). Additionally, the snapshot state may have changed. SeeNotReadableError
. -
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
the
get
stream
algorithm
to
fail
is
called
a
failure
reason
.
A
failure
reason
is
one
of
NotFound
,
UnsafeFile
,
TooManyReads
,
SnapshotState
,
or
FileLock
.
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.
Type | Description and Failure Reason |
---|---|
NotFoundError
|
If
the
File
or
Blob
resource
could
not
be
found
at
the
time
the
read
was
processed,
this
is
the
NotFound
failure
reason
.
For
asynchronous
read
methods
the
|
SecurityError
|
If:
For
asynchronous
read
methods
the
This is a security error to be used in situations not covered by any other failure reason . |
NotReadableError
|
If:
For
asynchronous
read
methods
the
|
8. A URL for Blob and MediaSource reference
This
section
defines
a
scheme
for
a
URL
used
to
refer
to
Blob
and
MediaSource
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
Blob
s
and
MediaSource
s
with
other
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(obj)
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
(of
type
Blob
or
MediaSource
),
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
.
-
Let result be the empty string.
-
Append the string "
blob:
" to result . -
Let settings be the current settings object
-
Let origin be settings ’s origin .
-
Let serialized be the ASCII serialization of origin .
-
If serialized is "
null
", set it to an implementation-defined value. -
Append serialized to result .
-
Append U+0024 SOLIDUS (
/
) to result . -
Generate a UUID [RFC4122] as a string and append it to result .
-
Return result .
blob:https://example.org/40a5fb5a-d56d-4a33-b4e2-0acf6a8e5f64
1.
-
Let store be the user agent’s blob URL store .
-
Let url be the result of generating a new blob URL .
-
Let entry be a new blob URL entry consisting of object and the current settings object .
-
Set store [ url ] to entry .
-
Return url .
-
Let store be the user agent’s blob URL store ;
-
Let url string be the result of serializing url .
-
Remove store [ url string ].
8.3. Dereferencing Model for blob URLs
-
Let store be the user agent’s blob URL store .
-
Let url string be the result of serializing url with the exclude fragment flag set.
-
If store [ url string ] exists , return store [ url string ]; otherwise return failure.
Futher requirements for the parsing and fetching model for blob URLs are defined in the [URL] and [Fetch] specifications.
8.3.1. Origin of blob URLs
This section is informative.
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. This is achieved by the [URL] spec looking up the URL in the blob URL store when parsing a URL, and using that entry to return the correct origin.
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.
8.3.2. Lifetime of blob URLs
This specification extends the unloading document cleanup steps with the following steps:
-
Let environment be the
Document
's relevant settings object . -
Let store be the user agent’s blob URL store ;
-
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.
8.4. 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 or MediaSource ));
obj static void revokeObjectURL (DOMString ); };
url
createObjectURL(
obj
)
static
method
must
return
the
result
of
adding
an
entry
to
the
blob
URL
store
for
obj
.
revokeObjectURL(
url
)
static
method
must
run
these
steps:
-
Let url record be the result of parsing url .
-
If url record ’s scheme is not "
blob
", return. -
Let origin be the origin of url record .
-
Let settings be the current settings object .
-
If origin is not same origin with settings ’s origin , return.
-
Remove an entry from the Blob URL Store for url .
Note: This means that rather than throwing some kind of error, attempting to revoke a URL that isn’t registered will silently fail. User agents might display a message on the error console if this happens.
Note: Attempts to dereference url after it has been revoked will result in a network error . Requests that were started before the url was revoked should still succeed.
window1
and
window2
are
separate,
but
in
the
same
origin
;
window2
could
be
an
iframe
inside
window1
.
myurl= window1. URL. createObjectURL( myblob); window2. URL. revokeObjectURL( myurl);
Since
a
user
agent
has
one
global
blob
URL
store
,
it
is
possible
to
revoke
an
object
URL
from
a
different
window
than
from
which
it
was
created.
The
URL.
call
ensures
that
subsequent
dereferencing
of
revokeObjectURL()
myurl
results
in
a
the
user
agent
acting
as
if
a
network
error
has
occurred.
8.4.1. Examples of blob URL Creation and Revocation
Blob
URL
s
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()
This section gives sample usage of creation and revocation of blob URL s with explanations.
img
elements
[HTML]
refer
to
the
same
blob
URL
:
url= URL. createObjectURL( blob); img1. src= url; img2. src= url;
URL.
revokeObjectURL()
is
explicitly
called.
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 theFileList
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 aSecurityError
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.
-
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. Thetext/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 thea
element initiates a download, saving aFile
with the name specified. The combination of this API and thedownload
attribute ona
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.
-
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, Jonas Sicking and Vsevolod Shmyroff 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