> ## 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.

# EventLog.getTopic0

> Get event signature hash (topic0)

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

<Tabs />

## Topic0 Semantics

**Non-anonymous events:** Topic0 is the keccak256 hash of the event signature:

```typescript theme={null}
import { Hash } from 'tevm';

// Transfer(address,address,uint256)
const transferSig = Hash(
  keccak256(new TextEncoder().encode('Transfer(address,address,uint256)'))
);

console.log(Hash.toHex(transferSig));
// 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
```

**Anonymous events:** Topic0 is the first indexed parameter (NOT the signature):

```solidity theme={null}
event AnonymousTransfer(
  address indexed from,
  address indexed to,
  uint256 indexed tokenId
) anonymous;
```

```typescript theme={null}
import { EventLog } from 'tevm';

const anonymousLog = EventLog({
  address: contractAddress,
  topics: [
    fromHash,    // topic0 = first indexed param (from)
    toHash,      // topic1 = second indexed param (to)
    tokenIdHash, // topic2 = third indexed param (tokenId)
  ],
  data: Bytes(),
});

const topic0 = anonymousLog.getTopic0();
// Returns fromHash, NOT event signature
```

## Usage Patterns

### Identifying Event Type

```typescript theme={null}
import { EventLog, Hash } from 'tevm';

const TRANSFER_SIG = Hash('0xddf252ad...');
const APPROVAL_SIG = Hash('0x8c5be1e5...');

function identifyEvent(log: EventLog): string {
  const sig = log.getTopic0();
  if (!sig) return 'unknown';

  if (Hash.equals(sig, TRANSFER_SIG)) return 'Transfer';
  if (Hash.equals(sig, APPROVAL_SIG)) return 'Approval';
  return 'unknown';
}

const eventType = identifyEvent(log);
console.log(`Event type: ${eventType}`);
```

### Filtering by Signature

```typescript theme={null}
import { EventLog, Hash } from 'tevm';

const TRANSFER_SIG = Hash('0xddf252ad...');

// Find all Transfer events
const transferLogs = allLogs.filter(log => {
  const sig = log.getTopic0();
  return sig && Hash.equals(sig, TRANSFER_SIG);
});

console.log(`Found ${transferLogs.length} Transfer events`);
```

### Validating Event Structure

```typescript theme={null}
import { EventLog, Hash } from 'tevm';

function validateTransferLog(log: EventLog): boolean {
  const sig = log.getTopic0();
  if (!sig) {
    console.error('Log missing signature');
    return false;
  }

  if (!Hash.equals(sig, TRANSFER_SIG)) {
    console.error('Not a Transfer event');
    return false;
  }

  if (log.getIndexedTopics().length !== 2) {
    console.error('Expected 2 indexed parameters');
    return false;
  }

  return true;
}
```

### Computing Event Signature

```typescript theme={null}
import { Hash } from 'tevm';

function eventSignature(declaration: string): Hash {
  const bytes = new TextEncoder().encode(declaration);
  return Hash(keccak256(bytes));
}

// ERC-20 events
const TRANSFER = eventSignature('Transfer(address,address,uint256)');
const APPROVAL = eventSignature('Approval(address,address,uint256)');

console.log('Transfer:', Hash.toHex(TRANSFER));
console.log('Approval:', Hash.toHex(APPROVAL));
```

## Empty Topics Array

If topics array is empty, `getTopic0()` returns `undefined`:

```typescript theme={null}
import { EventLog, Address } from 'tevm';

const emptyLog = EventLog({
  address: Address.zero(),
  topics: [],
  data: Bytes(),
});

const topic0 = emptyLog.getTopic0();
console.log(topic0); // undefined
```

## See Also

* [getSignature](/primitives/eventlog/getsignature) - Alias for getTopic0
* [getIndexedTopics](/primitives/eventlog/getindexedtopics) - Get topic1-topic3
* [Fundamentals](/primitives/eventlog/fundamentals) - Event signature hashing