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# Setting an Engine

## Overview

[`Engine.set`](/docs/engine/set) synchronously installs custom Engine slots. Slots and primitives
are optional, and repeated calls merge with the current overrides.

Use [`Engine.install`](/docs/engine/install) when an implementation requires asynchronous
initialization.

## Recipes

### Override One Primitive

Install a Node.js SHA-256 implementation while leaving every other primitive unchanged.

```ts twoslash
// @noErrors
import { createHash } from 'node:crypto'
import { Engine } from 'viem'
import { Hash } from 'viem/utils'

Engine.set({ // [!code focus]
  Hash: { // [!code focus]
    sha256: (input) => createHash('sha256').update(input).digest(), // [!code focus]
  }, // [!code focus]
}) // [!code focus]

const digest = Hash.sha256('0xdeadbeef')
// @log: '0x5f78c33274e43fa9de5659265c1d917e25c03722dcb0b8d27db8d5feaa813953'
```

### Merge Slots

Separate calls merge primitives instead of replacing the complete registry.

```ts twoslash
import { Engine } from 'viem'
import { Engine as NodeEngine } from 'viem/node'
import { Engine as WasmEngine } from 'viem/wasm'

const [node, wasm] = await Promise.all([
  NodeEngine.engine(),
  WasmEngine.engine(),
])

Engine.set({ Hash: node.Hash })
Engine.set({
  Mnemonic: wasm.Mnemonic, // [!code focus]
})
```

## `Engine.set`

Installs cryptographic implementations synchronously.

### Usage

```ts twoslash
// @noErrors
import { createHash } from 'node:crypto'
import { Engine } from 'viem'

Engine.set({
  Hash: {
    sha256: (input) => createHash('sha256').update(input).digest(),
  },
})
```

### Definition

```ts
function set(value: Engine.Engine): void
```

### Parameters

#### value

* **Type:** `Engine.Engine`

The Engine slots to install. Omitted slots and primitives preserve the current override or default.

```ts twoslash
// @noErrors
import { createHash } from 'node:crypto'
import { Engine } from 'viem'
// ---cut---
Engine.set({ // [!code focus]
  Hash: { // [!code focus]
    sha256: (input) => createHash('sha256').update(input).digest(), // [!code focus]
  }, // [!code focus]
}) // [!code focus]
```

### Return Value

`void`

No value is returned.

### Errors

| Error | Description |
| --- | --- |
| [`Engine.InvalidSlotValueError`](/docs/engine/errors#engineinvalidslotvalueerror) | A slot is not an object or `undefined`. |
| [`Engine.UnknownPrimitiveError`](/docs/engine/errors#engineunknownprimitiveerror) | A slot contains an unrecognized primitive. |
| [`Engine.UnknownSlotError`](/docs/engine/errors#engineunknownsloterror) | The Engine contains an unrecognized slot. |