components

4-binary/02-text-decoder/article.md

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+# TextDecoder and TextEncoder
+
+What if the binary data is actually a string? For instance, we received a file with textual data.
+
+The build-in [TextDecoder](https://encoding.spec.whatwg.org/#interface-textdecoder) object allows to read the value into an an actual Javascript string, given the buffer and the encoding.
+
+We first need to create it:
+```js
+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:
+
+```js
+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:
+
+```js run
+let uint8Array = new Uint8Array([72, 101, 108, 108, 111]);
+
+alert( new TextDecoder().decode(uint8Array) ); // Hello
+```
+
+
+```js run
+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:
+
+
+```js run
+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](https://encoding.spec.whatwg.org/#interface-textencoder) does the reverse thing -- converts a string into bytes.
+
+The syntax is:
+
+```js run
+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`.
+
+```js run
+let encoder = new TextEncoder();
+
+let uint8Array = encoder.encode("Hello");
+alert(uint8Array); // 72,101,108,108,111
+```