76 lines
2.4 KiB
Markdown
76 lines
2.4 KiB
Markdown
# TextDecoder and TextEncoder
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What if the binary data is actually a string? For instance, we received a file with textual data.
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The build-in [TextDecoder](https://encoding.spec.whatwg.org/#interface-textdecoder) object allows to read the value into an actual JavaScript string, given the buffer and the encoding.
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We first need to create it:
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```js
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let decoder = new TextDecoder([label], [options]);
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```
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- **`label`** -- the encoding, `utf-8` by default, but `big5`, `windows-1251` and many other are also supported.
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- **`options`** -- optional object:
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- **`fatal`** -- boolean, if `true` then throw an exception for invalid (non-decodable) characters, otherwise (default) replace them with character `\uFFFD`.
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- **`ignoreBOM`** -- boolean, if `true` then ignore BOM (an optional byte-order unicode mark), rarely needed.
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...And then decode:
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```js
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let str = decoder.decode([input], [options]);
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```
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- **`input`** -- `BufferSource` to decode.
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- **`options`** -- optional object:
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- **`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.
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For instance:
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```js run
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let uint8Array = new Uint8Array([72, 101, 108, 108, 111]);
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alert( new TextDecoder().decode(uint8Array) ); // Hello
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```
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```js run
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let uint8Array = new Uint8Array([228, 189, 160, 229, 165, 189]);
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alert( new TextDecoder().decode(uint8Array) ); // 你好
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```
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We can decode a part of the buffer by creating a subarray view for it:
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```js run
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let uint8Array = new Uint8Array([0, 72, 101, 108, 108, 111, 0]);
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// the string is in the middle
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// create a new view over it, without copying anything
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let binaryString = uint8Array.subarray(1, -1);
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alert( new TextDecoder().decode(binaryString) ); // Hello
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```
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## TextEncoder
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[TextEncoder](https://encoding.spec.whatwg.org/#interface-textencoder) does the reverse thing -- converts a string into bytes.
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The syntax is:
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```js
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let encoder = new TextEncoder();
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```
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The only encoding it supports is "utf-8".
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It has two methods:
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- **`encode(str)`** -- returns `Uint8Array` from a string.
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- **`encodeInto(str, destination)`** -- encodes `str` into `destination` that must be `Uint8Array`.
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```js run
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let encoder = new TextEncoder();
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let uint8Array = encoder.encode("Hello");
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alert(uint8Array); // 72,101,108,108,111
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```
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