What if the binary data is actually a string? For instance, we received a file with textual data.
The built-in TextDecoder object allows one to read the value into an actual JavaScript string, given the buffer and the encoding.
We first need to create it:
let decoder = new TextDecoder([label], [options]);
- label – the encoding, utf-8 by default, but big5, windows-1251 and many other are also supported.
- options – optional object:
- fatal – boolean, if true then throw an exception for invalid (non-decodable) characters, otherwise (default) replace them with character \uFFFD.
- ignoreBOM – boolean, if true then ignore BOM (an optional byte-order Unicode mark), rarely needed.
…And then decode:
let str = decoder.decode([input], [options]);
- input – BufferSource to decode.
- options – optional object:
- stream – true for decoding streams, when decoder is called repeatedly with incoming chunks of data. In that case a multi-byte character may occasionally split between chunks. This options tells TextDecoder to memorize “unfinished” characters and decode them when the next chunk comes.
For instance:
let uint8Array = new Uint8Array([72, 101, 108, 108, 111]);
alert( new TextDecoder().decode(uint8Array) ); // Hello
let uint8Array = new Uint8Array([228, 189, 160, 229, 165, 189]);
alert( new TextDecoder().decode(uint8Array) ); // 你好
We can decode a part of the buffer by creating a subarray view for it:
let uint8Array = new Uint8Array([0, 72, 101, 108, 108, 111, 0]);
// the string is in the middle
// create a new view over it, without copying anything
let binaryString = uint8Array.subarray(1, -1);
alert( new TextDecoder().decode(binaryString) ); // Hello
TextEncoder
TextEncoder does the reverse thing – converts a string into bytes.
The syntax is:
let encoder = new TextEncoder();
The only encoding it supports is “utf-8”.
It has two methods:
- encode(str) – returns Uint8Array from a string.
- encodeInto(str, destination) – encodes str into destination that must be Uint8Array.
let encoder = new TextEncoder();
let uint8Array = encoder.encode("Hello");
alert(uint8Array); // 72,101,108,108,111
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