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# Watch Token Transfers \[token.watchTransfer]

Watches TIP-20 `Transfer` events for a token.

## Usage

:::code-group
```ts twoslash [example.ts]
import { client } from './viem.config'

const watcher = client.token.watchTransfer({
  token: '0x20c0000000000000000000000000000000000000',
})

watcher.onLogs((logs) => {
  for (const log of logs) console.log(log.args)
  // @log: {
  // @log:   from: '0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266',
  // @log:   to: '0x70997970C51812dc3A010C7d01b50e0d17dc79C8',
  // @log:   amount: 100000000n
  // @log: }
})

// Later, tear down the watcher.
watcher.off()
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/tempo/viem.config.ts:setup]
```
:::

Also callable standalone: `Actions.token.watchTransfer(client, options)`, with `Actions` imported from `'viem/tempo'`.

## Recipes

### Credit Customer Deposits as They Arrive

Filter `args.to` to a deposit address to credit customer accounts on incoming transfers, formatting amounts for the ledger with [`Value.format`](/docs/utilities/value).

:::code-group
```ts twoslash [example.ts]
import { Value } from 'viem/utils'
import { client } from './viem.config'

const watcher = client.token.watchTransfer({
  args: { to: '0x742d35Cc6634C0532925a3b844Bc9e7595f0bEbb' }, // [!code focus]
  token: '0x20c0000000000000000000000000000000000000',
})

watcher.onLogs((logs) => {
  for (const log of logs) {
    // Credit the customer account mapped to this deposit address.
    console.log('Deposit:', log.args.from, Value.format(log.args.amount, 6)) // [!code focus]
    // @log: Deposit: 0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 249.99
  }
})
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/tempo/viem.config.ts:setup]
```
:::

### Sync an Offchain Ledger From a Checkpoint

Pass `fromBlock` when a ledger service restarts: transfers missed while it was offline replay first, then live events follow on the same stream.

:::code-group
```ts twoslash [example.ts]
import { client } from './viem.config'

// Last block the ledger recorded, loaded from its store.
const checkpoint = 1_204_320n

const watcher = client.token.watchTransfer({
  fromBlock: checkpoint + 1n, // [!code focus]
  token: '0x20c0000000000000000000000000000000000000',
})

for await (const { logs } of watcher) { // [!code focus]
  for (const log of logs) {
    // Record the transfer, then advance the checkpoint.
    console.log(log.blockNumber, log.args.from, log.args.to, log.args.amount)
  }
}
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/tempo/viem.config.ts:setup]
```
:::

## Return Value

Returns a watcher handle. Watching starts when the first listener (or async iterator) attaches, and stops when `off` is called.

```ts
type ReturnType = {
  /** Registers a log listener. Starts the watcher on first registration. Returns an unregister function. */
  onLogs: (fn: (logs: readonly Log[]) => void) => () => void
  /** Registers an error listener. Returns an unregister function. */
  onError: (fn: (error: Error) => void) => () => void
  /** Tears down the watcher: removes listeners, ends iterators, stops the poll or subscription. */
  off: () => void
  /** Async-iterates emitted log batches (latest-only stream). */
  [Symbol.asyncIterator]: () => AsyncIterableIterator<{ logs: readonly Log[] }>
}
```

Each log is decoded, with the event arguments on `args`:

```ts
type Log = {
  /** Decoded `Transfer` event arguments. */
  args: {
    /** Address that sent the tokens. */
    from: Address
    /** Address that received the tokens. */
    to: Address
    /** Amount transferred. */
    amount: bigint
  }
  /** Name of the emitted event. */
  eventName: 'Transfer'
  // ...standard log fields (address, blockHash, blockNumber, logIndex, transactionHash, ...)
}
```

## Parameters

### args

* **Type:** `object`

```ts
type Args = {
  /** Filter by sender address. */
  from?: Address | Address[] | null
  /** Filter by recipient address. */
  to?: Address | Address[] | null
}
```

Indexed argument values to filter logs by.

### batch

* **Type:** `boolean`
* **Default:** `true`

Whether to batch the logs found within a poll interval into a single emission. When `false`, each log is emitted on its own.

### fromBlock

* **Type:** `bigint`

Block number from which to start watching for logs.

### poll

* **Type:** `boolean`

Whether to poll for new logs instead of using a subscription. Defaults to `true` when the transport cannot subscribe or `fromBlock` is provided.

### pollingInterval

* **Type:** `number`
* **Default:** `client.pollingInterval`

Polling frequency (in ms).

### token

* **Type:** `Address | bigint`

Token to operate on: a TIP-20 token id or a contract address.