doc/api/string_decoder.md

# String decoder

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

<!-- source_link=lib/string_decoder.js -->

The `node:string_decoder` module provides an API for decoding `Buffer` objects
into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16
characters. It can be accessed using:

```mjs
import { StringDecoder } from 'node:string_decoder';
```

```cjs
const { StringDecoder } = require('node:string_decoder');
```

The following example shows the basic use of the `StringDecoder` class.

```mjs
import { StringDecoder } from 'node:string_decoder';
import { Buffer } from 'node:buffer';
const decoder = new StringDecoder('utf8');

const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent)); // Prints: ¢

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro)); // Prints: €
```

```cjs
const { StringDecoder } = require('node:string_decoder');
const decoder = new StringDecoder('utf8');

const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent)); // Prints: ¢

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro)); // Prints: €
```

When a `Buffer` instance is written to the `StringDecoder` instance, an
internal buffer is used to ensure that the decoded string does not contain
any incomplete multibyte characters. These are held in the buffer until the
next call to `stringDecoder.write()` or until `stringDecoder.end()` is called.

In the following example, the three UTF-8 encoded bytes of the European Euro
symbol (`€`) are written over three separate operations:

```mjs
import { StringDecoder } from 'node:string_decoder';
import { Buffer } from 'node:buffer';
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC]))); // Prints: €
```

```cjs
const { StringDecoder } = require('node:string_decoder');
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC]))); // Prints: €
```

## Class: `StringDecoder`

### `new StringDecoder([encoding])`

<!-- YAML
added: v0.1.99
-->

* `encoding` {string} The character [encoding][] the `StringDecoder` will use.
  **Default:** `'utf8'`.

Creates a new `StringDecoder` instance.

### `stringDecoder.end([buffer])`

<!-- YAML
added: v0.9.3
-->

* `buffer` {string|Buffer|TypedArray|DataView} The bytes to decode.
* Returns: {string}

Returns any remaining input stored in the internal buffer as a string. Bytes
representing incomplete UTF-8 and UTF-16 characters will be replaced with
substitution characters appropriate for the character encoding.

If the `buffer` argument is provided, one final call to `stringDecoder.write()`
is performed before returning the remaining input.
After `end()` is called, the `stringDecoder` object can be reused for new input.

### `stringDecoder.write(buffer)`

<!-- YAML
added: v0.1.99
changes:
  - version: v8.0.0
    pr-url: https://github.com/nodejs/node/pull/9618
    description: Each invalid character is now replaced by a single replacement
                 character instead of one for each individual byte.
-->

* `buffer` {string|Buffer|TypedArray|DataView} The bytes to decode.
* Returns: {string}

Returns a decoded string, ensuring that any incomplete multibyte characters at
the end of the `Buffer`, or `TypedArray`, or `DataView` are omitted from the
returned string and stored in an internal buffer for the next call to
`stringDecoder.write()` or `stringDecoder.end()`.

[encoding]: buffer.md#buffers-and-character-encodings