Chapter 2: Binary data, files

2.1 ArrayBuffer, binary arrays

ArrayBuffer is a reference to a fixed-length contiguous memory area.

let buffer = new ArrayBuffer(16);

It has a fixed length.
It takes exactly that much space in the memory.
To access individual bytes, another “view” object is needed, not buffer[index].

To manipulate an ArrayBuffer, we need to use a “view” object. A view object does not store anything on it’s own.

Uint8Array: 8-bit unsigned integer
Uint16Array: 16-bit unsigned integer
Uint32Array: 32-bit unsigned integer
Float64Array

let buffer = new ArrayBuffer(16);
let view = new Uint32Array(buffer);
view[0] = 123456;

TypedArray

Typed array is the common term for all these views, which behave like regular arrays: have indexes and iterable.

new TypedArray(buffer, [byteOffset], [length]);
new TypedArray(object); // array-like object
new TypedArray(typedArray); // copy and convert values
new TypedArray(length);
new TypedArray();

arr.buffer – references the ArrayBuffer.
arr.byteLength – the length of the ArrayBuffer.

Out-of-bounds behavior

If we attempt to write an out-of-bounds value into a typed array, extra bits on the left are cut-off.

TypedArray methods

No splice: we can't delete a value because they are contiguous areas of memory.
No concat
arr.set(fromArr, [offset]): copies all elements from fromArr to the arr, starting at position offset
arr.subarray([begin, end]): creates a new view of the same type from begin to end

DataView

DataView is a special super-flexible untyped view over ArrayBuffer. It allows to access the data on any offset in any format.

new DataView(buffer, [byteOffset], [byteLength])

With DataView we can access the data with methods like .getUint8(i) or .getUint16(i). It is great when we store mixed-format data in the same buffer.

2.2 TextDecoder and TextEncoder

The build-in TextDecoder object allows to read the value into an actual string, given the buffer and the encoding.

TextDecoder

let decoder = new TextDecoder([label], [options]);

label: the encoding, utf-8 by default
options
- fatal: boolean, if true then throw an exception for invalid characters
- ignoreBOM: boolean, if true then ignore BOM

let str = decoder.decode([input], [options]);

input: BufferSource to decode
options
- stream: true for decoding streams, when decoder is called repeatedly with incoming chunks of data.

TextEncoder

TextEncoder converts a string into bytes.

let encoder = new TextEncoder();

encode(str): returns Uint8Array from a string
encodeInto(str, destination): encodes str into destination that is the Uint8Array

2.3 Blob

Blob consists of an optional string type (a MIME-type usually), plus blobParts – a sequence of other Blob objects, strings and BufferSource.

new Blob(blobParts, options);

blobParts: an array of Blob/BufferSource/String
options
- type: Blog type
- endings: whether to transform end-of-line

let blob = new Blob(["<html>…</html>"], {type: 'text/html'});

Extract

blob.slice([byteStart], [byteEnd], [contentType]);

We can’t change data directly in a Blob, but we can slice it and create new Blob objects from them.

Blob as URL

URL.createObjectURL takes a Blob and creates a unique URL for it, in the form blob:<origin>/<uuid>.

let blob = new Blob(["Hello, world!"], {type: 'text/plain'});

link.href = URL.createObjectURL(blob);

A generated URL is only valid within the current document, while it’s open. If we create a URL, that Blob will hang in memory, even if not needed any more.

URL.revokeObjectURL(url) removes the reference from the internal mapping.

Blob to base64

Base64 represents binary data as a string of ultra-safe readable characters with ASCII-codes from 0 to 64.

A data url has the form data:[<mediatype>][;base64],<data>.

<img src="data:image/png;base64,R0lGODlhDAAMAKIFAF5LAP/zxAAAANyuAP/gaP///wAAAAAAACH5BAEAAAUALAAAAAAMAAwAAAMlWLPcGjDKFYi9lxKBOaGcF35DhWHamZUW0K4mAbiwWtuf0uxFAgA7">

Image to blob

Image operations are done via <canvas> element.

From Blob to ArrayBuffer

let fileReader = new FileReader();
fileReader.readAsArrayBuffer(blob);

2.4 File and FileReader

File

A File object inherits from Blob and is extended with filesystem-related capabilities.

new File(fileParts, fileName, [options])

options
- lastModified

FileReader

let reader = new FileReader();

readAsArrayBuffer(blob)
readAsText(bot, encoding)
readAsDataURL(blob)
reader.error is the error.
FileReaderSync is available inside Web Workers.

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