> ## 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.

# Selector

> 4-byte function selector for Ethereum function signatures

<Card title="Try it Live" icon="play" href="https://playground.tevm.sh?example=primitives/selector.ts">
  Run Selector examples in the interactive playground
</Card>

# Selector

A **Selector** is a 4-byte identifier used to uniquely identify Ethereum function calls. It's computed as the first 4 bytes of the Keccak-256 hash of the function signature.

## Type Definition

```typescript theme={null}
type SelectorType = Uint8Array & {
  readonly [brand]: "Selector";
  readonly length: 4;
};
```

## Creating Selectors

### From Signature String

The most common way to create a selector is from a function signature:

```typescript theme={null}
import * as Selector from '@tevm/primitives';

const transferSelector = Selector.fromSignature('transfer(address,uint256)');
console.log(transferSelector.toHex()); // '0xa9059cbb'
```

### From Hex String

```typescript theme={null}
const selector = Selector.fromHex('0xa9059cbb');
```

### From Bytes

```typescript theme={null}
const bytes = new Uint8Array([0xa9, 0x05, 0x9c, 0xbb]);
const selector = Selector.from(bytes);
```

## Operations

### Convert to Hex

```typescript theme={null}
const hex = selector.toHex();
// '0xa9059cbb'
```

### Compare Selectors

```typescript theme={null}
const sel1 = Selector.fromSignature('transfer(address,uint256)');
const sel2 = Selector.fromSignature('approve(address,uint256)');
const equal = sel1.equals(sel2); // false
```

## Common Selectors

### ERC-20

* `transfer(address,uint256)` → `0xa9059cbb`
* `approve(address,uint256)` → `0x095ea7b3`
* `balanceOf(address)` → `0x70a08231`
* `totalSupply()` → `0x18160ddd`

### ERC-721

* `transferFrom(address,address,uint256)` → `0x23b872dd`
* `safeTransferFrom(address,address,uint256)` → `0x42842e0e`
* `ownerOf(uint256)` → `0x6352211e`

## Signature Format

Function signatures must use **canonical type names**:

✅ Correct:

* `transfer(address,uint256)`
* `swap(uint256,uint256,address,bytes)`
* `execute((address,uint256,bytes)[])`

❌ Incorrect:

* `transfer(address, uint256)` (has spaces)
* `transfer(address,uint)` (should be uint256)
* `Transfer(address,uint256)` (wrong capitalization)

## How Selectors Work

1. **Function Signature**: Start with the canonical function signature
2. **Hash**: Compute `keccak256(signature)`
3. **Truncate**: Take the first 4 bytes

```
transfer(address,uint256)
  ↓ keccak256
0xa9059cbb2fead7aba11f5e0a11af...
  ↓ slice(0, 4)
0xa9059cbb
```

## Selector Collisions

While rare, different function signatures can theoretically produce the same selector (collision). In practice, this is extremely unlikely due to the cryptographic properties of Keccak-256.

If a collision occurs:

1. One function will override the other in the contract
2. The EVM will use whichever function is defined first
3. This is considered a critical contract bug

## Usage in Contracts

Selectors are used in:

1. **Function Calls**: First 4 bytes of transaction data identify the function
2. **Low-level Calls**: `address.call(abi.encodeWithSelector(selector, args))`
3. **Function Routing**: Contracts use selectors to route calls to the correct function

## API Reference

### Constructors

* `from(value: SelectorLike): SelectorType` - Create from various inputs
* `fromHex(hex: string): SelectorType` - Create from hex string
* `fromSignature(signature: string): SelectorType` - Compute from function signature

### Operations

* `toHex(selector: SelectorType): string` - Convert to hex string
* `equals(a: SelectorType, b: SelectorType): boolean` - Compare selectors

## See Also

* [Selector (Effect)](https://voltaire-effect.tevm.sh/primitives/selector) - Effect.ts integration with Schema validation
* [FunctionSignature](/primitives/function-signature) - Extended selector with metadata
* [EventSignature](/primitives/event-signature) - 32-byte event topic
* [ErrorSignature](/primitives/error-signature) - 4-byte error selector
* [ABI Encoding](/primitives/abi) - Encoding function calls