> ## Documentation Index
> Fetch the complete documentation index at: https://voltaire.tevm.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# ChainId

> EIP-155 chain identifier for replay protection

## Overview

ChainId is a branded number type representing an EIP-155 chain identifier. Used for replay protection and identifying which network a transaction targets.

```typescript theme={null}
import * as ChainId from '@voltaire/primitives/ChainId'

const mainnet = ChainId.from(1)
const sepolia = ChainId.from(11155111)

if (ChainId.isMainnet(1)) {
  console.log('Using Ethereum mainnet')
}
```

## Type Definition

```typescript theme={null}
type ChainIdType = number & { readonly [brand]: "ChainId" }
```

The branded type prevents accidentally mixing chain IDs with other numbers.

## Creating ChainId

### from

Create from a non-negative integer.

```typescript theme={null}
const mainnet = ChainId.from(1)
const arbitrum = ChainId.from(42161)
const base = ChainId.from(8453)
```

**Validation:** Throws `InvalidFormatError` if value is not a non-negative integer.

```typescript theme={null}
ChainId.from(-1)    // Error: must be non-negative
ChainId.from(1.5)   // Error: must be integer
```

## Constants

Pre-defined chain IDs for common networks:

```typescript theme={null}
import { MAINNET, SEPOLIA, HOLESKY, OPTIMISM, ARBITRUM, BASE, POLYGON } from '@voltaire/primitives/ChainId'

// Ethereum
const mainnet = MAINNET      // 1
const sepolia = SEPOLIA      // 11155111
const holesky = HOLESKY      // 17000

// Layer 2s
const optimism = OPTIMISM    // 10
const arbitrum = ARBITRUM    // 42161
const base = BASE            // 8453
const polygon = POLYGON      // 137
```

<Note>
  `GOERLI` (5) is deprecated. Use `SEPOLIA` or `HOLESKY` for testnet development.
</Note>

## Methods

### toNumber

Convert to plain number.

```typescript theme={null}
const n = ChainId.toNumber(42161) // 42161
```

### equals

Compare two chain IDs.

```typescript theme={null}
const same = ChainId.equals(1, 1)           // true
const different = ChainId.equals(1, 10)     // false
```

### isMainnet

Check if chain ID is Ethereum mainnet.

```typescript theme={null}
ChainId.isMainnet(1)     // true
ChainId.isMainnet(10)    // false (Optimism)
ChainId.isMainnet(42161) // false (Arbitrum)
```

## Common Chain IDs

| Chain        | ID       | Type      |
| ------------ | -------- | --------- |
| Ethereum     | 1        | Mainnet   |
| Sepolia      | 11155111 | Testnet   |
| Holesky      | 17000    | Testnet   |
| Optimism     | 10       | L2        |
| Arbitrum One | 42161    | L2        |
| Base         | 8453     | L2        |
| Polygon      | 137      | Sidechain |
| Avalanche    | 43114    | L1        |
| BSC          | 56       | L1        |

## EIP-155 Replay Protection

Chain ID is embedded in transaction signatures to prevent replay attacks across chains:

```typescript theme={null}
import * as ChainId from '@voltaire/primitives/ChainId'

const tx = {
  chainId: ChainId.from(1),  // Mainnet only
  to: "0x...",
  value: 1000000000000000000n,
  // ... other fields
}

// This transaction can only be valid on Ethereum mainnet
// Submitting to Arbitrum would fail signature verification
```

## Namespace Usage

```typescript theme={null}
import { ChainId } from '@voltaire/primitives/ChainId'

// Using namespace object
const id = ChainId.from(1)
const isMain = ChainId.isMainnet(1)
const equal = ChainId.equals(1, 1)
```

## See Also

* [Chain](/primitives/chain) - Full chain metadata
* [Transaction](/primitives/transaction) - Transaction signing
* [EIP-155](https://eips.ethereum.org/EIPS/eip-155) - Replay protection spec