Skip to main content

Try it Live

Run Selector examples in the interactive playground

​
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

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: 
import * as Selector from '@tevm/primitives';

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

​
From Hex String

const selector = Selector.fromHex('0xa9059cbb');

​
From Bytes

const bytes = new Uint8Array([0xa9, 0x05, 0x9c, 0xbb]);
const selector = Selector.from(bytes);

​
Operations

​
Convert to Hex

const hex = selector.toHex();
// '0xa9059cbb'

​
Compare Selectors

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