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EventSignature

An EventSignature is a 32-byte identifier used as the first topic (topic[0]) in Ethereum event logs. It’s computed as the full Keccak-256 hash of the event signature.

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Type Definition

type EventSignatureType = Uint8Array & {
  readonly [brand]: "EventSignature";
  readonly length: 32;
};

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Creating Event Signatures

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From Signature String

import * as EventSignature from '@tevm/primitives';

const transferEvent = EventSignature.fromSignature(
  'Transfer(address,address,uint256)'
);

console.log(EventSignature.toHex(transferEvent));
// '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef'

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From Hex String

const sig = EventSignature.fromHex(
  '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef'
);

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From Bytes

const bytes = new Uint8Array(32); // 32-byte array
const sig = EventSignature.from(bytes);

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Operations

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Convert to Hex

const hex = EventSignature.toHex(signature);
// '0xddf252ad...'

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Compare Signatures

const sig1 = EventSignature.fromSignature('Transfer(address,address,uint256)');
const sig2 = EventSignature.fromSignature('Approval(address,address,uint256)');
const equal = EventSignature.equals(sig1, sig2); // false

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Common Event Signatures

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ERC-20

// Transfer(address indexed from, address indexed to, uint256 value)
const transfer = EventSignature.fromSignature('Transfer(address,address,uint256)');
// 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef

// Approval(address indexed owner, address indexed spender, uint256 value)
const approval = EventSignature.fromSignature('Approval(address,address,uint256)');
// 0x8c5be1e5ebec7d5bd14f71427d1e84f3dd0314c0f7b2291e5b200ac8c7c3b925

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ERC-721

// Transfer(address indexed from, address indexed to, uint256 indexed tokenId)
const transfer = EventSignature.fromSignature('Transfer(address,address,uint256)');
// Same signature as ERC-20 Transfer!

// Approval(address indexed owner, address indexed approved, uint256 indexed tokenId)
const approval = EventSignature.fromSignature('Approval(address,address,uint256)');
// Same signature as ERC-20 Approval!

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Uniswap V2

// Swap(address indexed sender, uint256 amount0In, uint256 amount1In,
//      uint256 amount0Out, uint256 amount1Out, address indexed to)
const swap = EventSignature.fromSignature(
  'Swap(address,uint256,uint256,uint256,uint256,address)'
);
// 0xd78ad95fa46c994b6551d0da85fc275fe613ce37657fb8d5e3d130840159d822

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Event Log Structure

Event signatures appear as the first topic in event logs: 
{
  topics: [
    '0xddf252ad...',  // topic[0] = event signature
    '0x000...sender', // topic[1] = indexed param 1
    '0x000...recipient' // topic[2] = indexed param 2
  ],
  data: '0x000...amount' // non-indexed params
}

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Signature Format

Event signatures must use canonical type names: ✅ Correct:
  • Transfer(address,address,uint256)
  • Swap(address,uint256,uint256,uint256,uint256,address)
  • Deposit(address,(uint256,uint256))
❌ Incorrect:
  • Transfer(address, address, uint256) (has spaces)
  • Transfer(address,address,uint) (should be uint256)
  • transfer(address,address,uint256) (wrong capitalization)
Note: Unlike function signatures, event names conventionally start with uppercase.

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How Event Signatures Work

  1. Event Signature: Start with the canonical event signature
  2. Hash: Compute keccak256(signature)
  3. Use Full Hash: Unlike function selectors, use all 32 bytes
Transfer(address,address,uint256)
  ↓ keccak256
0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
  ↓ (no truncation)
0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef

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Indexed vs Non-Indexed Parameters

Event parameters can be indexed or non-indexed: 
event Transfer(
  address indexed from,   // topic[1]
  address indexed to,     // topic[2]
  uint256 value           // data
);
The signature does not include the indexed keyword:
  • Signature: Transfer(address,address,uint256)
  • The indexed keyword only affects where data appears in the log

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Anonymous Events

Anonymous events don’t include the event signature as topic[0]: 
event Transfer(
  address indexed from,
  address indexed to,
  uint256 value
) anonymous;
For anonymous events:
  • No topic[0] (no event signature)
  • Indexed params use topic[0], topic[1], topic[2]…
  • Saves gas but harder to identify the event

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Filtering Events

Use event signatures to filter logs: 
const transferSig = EventSignature.fromSignature('Transfer(address,address,uint256)');

const logs = await eth_getLogs({
  address: tokenAddress,
  topics: [EventSignature.toHex(transferSig)]
});

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API Reference

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Constructors

  • from(value: EventSignatureLike): EventSignatureType - Create from various inputs
  • fromHex(hex: string): EventSignatureType - Create from hex string
  • fromSignature(signature: string): EventSignatureType - Compute from event signature

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Operations

  • toHex(sig: EventSignatureType): string - Convert to hex string
  • equals(a: EventSignatureType, b: EventSignatureType): boolean - Compare signatures

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See Also