---
title: "@unikvs/zstd.wasm"
description: "Middleware that compresses and decompresses with a WebAssembly Zstandard implementation."
---

## Overview [#overview]

`@unikvs/zstd.wasm` is a plugin that compresses bytes with Zstandard for storage and decompresses them on read. Middleware

It lazily resolves `@bokuweb/zstd-wasm` via dynamic `import` and initializes it before use. Use it when you want Zstandard regardless of runtime.

- `name` is `ZstdWasm`.
- `isOpen` becomes `true` after `open()`. It is `false` initially.
- Streaming `encodeStream` and `decodeStream` are not provided. Only batched conversion is supported.

Install with:

```package-install
npm install @unikvs/zstd.wasm @logtape/logtape
```

## Usage [#usage]

`ZstdWasm` implements IMiddleware. Pass it to UniKvs with use().

```ts
import { ZstdWasm } from "@unikvs/zstd.wasm";
```

Constructor arguments:

| Prop | Type | Default | Description |
| - | - | - | - |
| `level?` | `[#level] number \| undefined` | - | Compression level. Specify an integer from 1 to 22. Higher values compress better but take longer. Defaults to 3 when omitted. |

The following example compresses written values with Zstandard for storage and decompresses them on read.

```ts
import { Memory } from "@unikvs/memory";
import * as unikvs from "unikvs";
import { ZstdWasm } from "@unikvs/zstd.wasm";

const kvs = new unikvs.Builder<{ foo: unikvs.Value<Uint8Array> }>()
  .use(new ZstdWasm({ level: 3 }))
  .persist(new Memory())
  .build();

await kvs.open();
await kvs.set("foo", new Uint8Array([0, 1, 2]));

const bytes = await kvs.get("foo");
console.log(bytes); // Decompressed bytes

await kvs.close();
```

## Data types [#data-types]

| Method   | Input                     | Output                    |
| -------- | ------------------------- | ------------------------- |
| `encode` | `Uint8Array<ArrayBuffer>` | `Uint8Array<ArrayBuffer>` |
| `decode` | `Uint8Array<ArrayBuffer>` | `Uint8Array<ArrayBuffer>` |

- `encode` compresses input bytes with Zstandard.
- `decode` decompresses input bytes with Zstandard.
- Streaming `encodeStream` and `decodeStream` are not provided.

## Errors [#errors]

Errors thrown by this package:

### `ZstdWasmInitError` [#error-zstd-wasm-init]

#### Meaning

Thrown when `open()` fails to initialize the `@bokuweb/zstd-wasm` module.

#### How to fix

Check that `@bokuweb/zstd-wasm` is installed correctly and that the browser can fetch the WASM file.

#### Example

The following example catches `ZstdWasmInitError`.

```ts
import { Memory } from "@unikvs/memory";
import * as unikvs from "unikvs";
import { ZstdWasm, ZstdWasmInitError } from "@unikvs/zstd.wasm";

const kvs = new unikvs.Builder<{ foo: unikvs.Value<Uint8Array> }>()
  .use(new ZstdWasm())
  .persist(new Memory())
  .build();

try {
  await kvs.open();
} catch (ex) {
  if (ex instanceof ZstdWasmInitError) {
    console.log("Failed to initialize");
  } else {
    throw ex;
  }
}
```

### `ZstdWasmInvalidLevelError` [#error-zstd-wasm-invalid-level]

| Prop | Type | Default | Description |
| - | - | - | - |
| `actual` | `[#zstd-wasm-invalid-level-actual] unknown` | - | The specified compression level. |

#### Meaning

The constructor throws this error when `level` is not an integer from 1 to 22.

#### How to fix

Specify an integer from 1 to 22. It defaults to 3 when omitted.

#### Example

The following example catches `ZstdWasmInvalidLevelError`.

```ts
import { ZstdWasm, ZstdWasmInvalidLevelError } from "@unikvs/zstd.wasm";

try {
  new ZstdWasm({ level: 0 });
} catch (ex) {
  if (ex instanceof ZstdWasmInvalidLevelError) {
    console.log(ex.meta); // { actual: 0 }
  } else {
    throw ex;
  }
}
```

Calling `encode` or similar before `open()` throws a `TypeError`. Decompression failures propagate the upstream error unchanged.