# Interact with ERC-20 tokens
> Interact with ERC 20 tokens
# Interact with ERC-20 tokens
[ERC-20](https://eips.ethereum.org/EIPS/eip-20) is a simple token standard and the most common contract type on Ethereum.
You can:
- [Send ERC-20 transactions](#send-transactions) using `eth_sendRawTransaction`.
- [Observe event logs of mined ERC-20 transactions](#observe-logs-of-mined-transactions) using `eth_getLogs`.
- Follow [this tutorial](../tutorials/ethereum/retrieve-the-balance-of-an-erc-20-token.md) to retrieve the balance of ERC-20 tokens.
- Follow [this tutorial](../tutorials/ethereum/track-erc-20-token-transfers.md) to track ERC-20 token transfers.
## ERC-20 token functions and events
An ERC-20 token must implement the following functions:
- `totalSupply()` - Returns the total token supply.
- `balanceOf(owner)` - Returns the account balance of another account with address `owner`.
- `allowance(owner, spender)` - Returns the amount which `spender` is still allowed to withdraw from `owner`.
- `transfer(to, value)` - Transfers `value` amount of tokens to address `to.`
- `approve(spender, value)` - Allows `spender` to withdraw from your account multiple times, up to the `value` amount.
- `transferFrom(from, to, value)` - Transfers `value` amount of tokens from address `from` to address `to`.
:::tip
Some widely used tokens don't strictly follow ERC-20 return conventions. Don't
assume `transfer` or `transferFrom` return a boolean value. Use a safe wrapper
(such as [OpenZeppelin's `SafeERC20`](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/utils/SafeERC20.sol)) or explicitly handle empty return data.
:::
At certain times, an ERC-20 token also must emit the following events:
- `Transfer(from, to, value)` - Must trigger when tokens are transferred, including zero value transfers.
- `Approval(owner, spender, value)` - Must trigger on any successful call to `approve(spender, value)`.
View [EIP-20](https://eips.ethereum.org/EIPS/eip-20) for more details about how these functions work and when to emit these events.
## Send transactions
Use [`eth_sendRawTransaction`](../reference/ethereum/json-rpc-methods/eth_sendrawtransaction.mdx) to send ERC-20 token transactions.
The JSON-RPC format expects `eth_sendRawTransaction` to have a specific data field format that requires normalizing the `transfer` function to a short [function selector](https://solidity.readthedocs.io/en/develop/abi-spec.html#function-selector). To do this, set the parameters for the function and run it through Ethereum's [sha3 keccak hash](https://solidity.readthedocs.io/en/develop/abi-spec.html#function-selector):
```javascript
web3.utils.sha3('transfer(address,uint256)').slice(0, 10)
```
```javascript
0xa9059cbb
```
The first four bytes of this hash comprise its four-byte signature. Take this four-byte signature, pad it with zeros, and package this information into a data string. Then sign the transaction and send it using `eth_sendRawTransaction`:
```bash
curl https://mainnet.infura.io/v3/ \
-X POST \
-H "Content-Type: application/json" \
-d '{"jsonrpc": "2.0", "method": "eth_sendRawTransaction", "params": ["0xf869018203e882520894f17f52151ebef6c7334fad080c5704d77216b732881bc16d674ec80000801ba02da1c48b670996dcb1f447ef9ef00b33033c48a4fe938f420bec3e56bfd24071a062e0aa78a81bf0290afbc3a9d8e9a068e6d74caa66c5e0fa8a46deaae96b0833"], "id": 1}'
```
```json
{
"id": 1,
"jsonrpc": "2.0",
"result": "0xe670ec64341771606e55d6b4ca35a1a6b75ee3d5145a99d05921026d1527331"
}
```
## Observe logs of mined transactions
When a transaction is mined, event logs are published for public viewing.
Once the event logs are published, you can execute [`eth_getLogs`](../reference/ethereum/json-rpc-methods/eth_getlogs.mdx) to investigate what changed relative to the events that you care about, and react to them.
:::success
For example, an event ticketing service that wants to issue offchain tickets based on crypto payments can use `eth_getLogs` to find payments to their address, and react to these events by processing some logic in their backend servers to issue tickets to users.
:::
The following example uses `eth_getLogs` on the DAI ERC-20 Solidity contract [`0x6B175474E89094C44Da98b954EedeAC495271d0F`](https://etherscan.io/address/0x6b175474e89094c44da98b954eedeac495271d0f#code):
```bash
curl https://mainnet.infura.io/v3/ \
-X POST \
-H "Content-Type: application/json" \
-d '{"jsonrpc": "2.0", "method": "eth_getLogs", "id": 1, "params": [{"fromBlock": "0x91F37C", "toBlock": "0x91F37C", "topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x000000000000000000000000ee25e1ba53c225d250861c8e5a9a3e0fe19c790e", "0x000000000000000000000000dfbaf3e4c7496dad574a1b842bc85b402bdc298d"], "address": "0x6B175474E89094C44Da98b954EedeAC495271d0F"}]}'
```
```json
{
"jsonrpc": "2.0",
"id": 1,
"result": [
{
"address": "0x6B175474E89094C44Da98b954EedeAC495271d0F",
"topics": [
"0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
"0x000000000000000000000000ee25e1ba53c2250250861c8e5a9a3e0fe19c790e",
"0x000000000000000000000000dfbaf3e4c7496dad574a1b842bc85b402bdc298d"
],
"data": "0x00000000000000000000000000000000000000000000041f900d25d6693623a6",
"blockNumber": "0x91F37C"
}
]
}
```
In this example request, the parameters `fromBlock` and `toBlock` specify the hexadecimal block number to retrieve logs from.
:::info
If `fromBlock` and `toBlock` are omitted, `eth_getLogs` returns the _entire_ chain history by default. Infura has a cap on requests of 10,000 events per query. We recommend requesting a single block, as in this example, and to do that for each mined block.
:::
This request tells the blockchain to retrieve event logs related to address `0x6B175474E89094C44Da98b954EedeAC495271d0F` emitted in block `0x91F37C` that match topics `0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef`, `0x000000000000000000000000ee25e1ba53c225d250861c8e5a9a3e0fe19c790e` and `0x000000000000000000000000dfbaf3e4c7496dad574a1b842bc85b402bdc298d`.
The response returned for this request is an array of events. In this example, only one event for one address matches the specified topics.
### Topics
Topics are events emitted by smart contracts. Looking at the source code of the original contract [`0x6B175474E89094C44Da98b954EedeAC495271d0F`](https://etherscan.io/address/0x6b175474e89094c44da98b954eedeac495271d0f#code) used in this example, there are two event signatures that could be associated with it on lines 94 and 95:
```solidity
event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
event Transfer(address indexed from, address indexed to, uint tokens);
```
To find out which topic (event) it actually was, create the [function selector](https://docs.soliditylang.org/en/develop/abi-spec.html#function-selector) of the event and take the [sha3 keccak hash](https://docs.soliditylang.org/en/develop/abi-spec.html#function-selector) of it. Let's try the event on line 94:
```javascript
web3.sha3('Approval(address,address,uint256)')
```
```javascript
0x8c5be1e5ebec7d5bd14f71427d1e84f3dd0314c0f7b2291e5b200ac8c7c3b925
```
The resulting hash doesn't match the hash provided in the initial request response. Now let's try the event on line 95 of the contract:
```javascript
web3.sha3('Transfer(address,address,uint256)')
```
```javascript
0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
```
The resulting hash matches the hash provided in the initial request response. Now you know that `0xddf25` is the transfer event in this example.
### Data
The `data` field in the request response refers to all the "non-indexed stuff" captured in the events. In this example, for the transfer topic, `data` represents the number of tokens that were transferred. That is, `0x41f900d25d6693623a6` or 19471.6949921 DAI tokens were transferred from `ee25e1ba53c225d250861c8e5a9a3e0fe19c790e` to `dfbaf3e4c7496dad574a1b842bc85b402bdc298d`.