How to create a token sponsor (ERC-20)

In most of the how to guides in the Core SDK, we provide a gasless sponsor that entirely pays for transactions on behalf of FunWallets. A Fun token sponsor does not entirely pay for transactions, instead it enables you to pay gas using ERC-20 tokens.

The Fun token sponsor works by having a supply of ETH that it can use to pay for transactions on your behalf. In exchange, it will withdraw an appropriate amount of a specific ERC-20 token from your FunWallet. This process lets you pay for a transaction using ERC-20s and in this guide, I’ll show you how to create your own token sponsor. I’ll also reference information covered in the Quick Start but I’ll quickly walk through all the steps again.

import {
  FunWallet,
  configureEnvironment,
  Auth,
  TokenSponsor,
  generatePrivateKey,
} from "@funkit/core";

To begin, we need to set up the environment. We’ll be using the Goerli testnet and a rate-limited API key. Please visit our dashboard to receive your own production apiKey.

await configureEnvironment({
  chain: "goerli",
  apiKey: "ZrhepzWGxm74D0sqstuhT6dGrJxhoy8SZIToX6I3",
});

We’ll also initialize a FunWallet here that will use the token sponsor once we create it.

const privateKeyWallet = generatePrivateKey();
const funWalletAuth = new Auth({ privateKey: privateKeyWallet });
const funWallet = new FunWallet({
  users: [{ userId: await funWalletAuth.getAddress() }],
  uniqueId: await funWalletAuth.getWalletUniqueId(),
});

In order to create a token sponsor, we’ll first have to create its address. I’ll derive one from my private key.

const privateKey = generatePrivateKey();
const sponsor = new Auth({ privateKey });
const sponsorAddress = await sponsor.getAddress();

We’ll initialize the token sponsor with this private key later but for now we can actually use the sponsorAddress to re-configure environment so that it has a token sponsor. Let me quickly explain token sponsor parameters.

It is important to note that token sponsor is not ready for use yet, we still have to configure all the following steps before transaction can be paid for in DAI.

await configureEnvironment({
  chain: "goerli",
  apiKey: "ZrhepzWGxm74D0sqstuhT6dGrJxhoy8SZIToX6I3",
  gasSponsor: {
    sponsorAddress,
    token: "0x0a1F0598A561af6b84A726bE007f581E812C3CDD",
    usePermit: true,
  },
});

After re-configuring the environment, we will create the token sponsor interface and use it to deposit (stake) ETH into the sponsor. The token sponsor will use this ETH to pay for transactions on behalf of other wallets. Let me explain the staking parameters.

Since we’re using funds in our private key to deposit ETH into the sponsor, that will be our depositor. We’ve also set our address derived from our private key as the token sponsor. This means the public address of the private key can be called to sponsor gas transactions.

// Create token sponsor interface
const tokenSponsor = new TokenSponsor();

// Stake native gas tokens to the gas sponsor
const depositAmount = 0.005;
const depositorAddress = sponsorAddress;
const depositEth = await tokenSponsor.stake(
  depositorAddress,
  sponsorAddress,
  depositAmount
);

With a Fun sponsor, you have the ability to tune who uses it and which tokens are allowed and in this section I’ll show you the different methods to do that. However I’ll also recommend you check out How to whitelist users from a gas sponsor or How to whitelist tokens from a token sponsor for more details.

First, we will set the gas sponsor to whitelist mode. Whitelist mode means you choose specific wallets that are allowed to use the gas sponsor. This is useful if you want to control how many wallets can use the sponsor. For instance, you might decide to let only users who paid for a subscription use the sponsor - in this situation you want to use whitelist mode.

const whitelistMode = await tokenSponsor.setToWhitelistMode(5, sponsorAddress)const whitelistUser = await tokenSponsor.addSpenderToWhiteList(5, sponsorAddress, await funWallet.getAddress())

Second, we will enable DAI as a token that can be used for gas payments on the sponsor.

const whitelistToken = await tokenSponsor.batchWhitelistTokens(
  sponsorAddress,
  ["0x0a1F0598A561af6b84A726bE007f581E812C3CDD"],
  [true]
);

Finally, we will whitelist our FunWallet, so that our FunWallet can use the sponsor.

const whitelistUser = await tokenSponsor.addSpenderToWhiteList(
  5,
  sponsorAddress,
  await funWallet.getAddress()
);
//Send all transactions for execution
await sponsor.sendTxs([
  depositEth,
  whitelistToken,
  whitelistMode,
  whitelistUser,
]);

Now that we have created a DAI token sponsor, the FunWallet we created at the start of this guide is set up to use DAI for gas. First we’ll add some custom DAI token to our FunWallet.

const transferTokensToWallet = await new Token(
  "0x0a1F0598A561af6b84A726bE007f581E812C3CDD"
).transfer(await funWallet.getAddress(), 100);
await sponsor.sendTxs([transferTokensToWallet]);

Then execute a transaction. Note that I’m executing a transaction like normal - this is because we have already set up our environment to use the token sponsor.

const op = await funWallet.create(funWalletAuth, await sponsor.getAddress());
await funWallet.executeOperation(funWalletAuth, op);

Putting together all the pieces we have createTokenSponsor() that you can use to create a token sponsor and execute a transaction using it.

import { FunWallet, configureEnvironment, Auth, TokenSponsor, Token, generatePrivateKey } from "@funkit/core"

const createTokenSponsor = async () => {
await configureEnvironment({
chain: "goerli",
apiKey: "ZrhepzWGxm74D0sqstuhT6dGrJxhoy8SZIToX6I3",
})

const privateKeyWallet = generatePrivateKey()
const funWalletAuth = new Auth({ privateKey: privateKeyWallet })
const funWallet = new FunWallet({
users: [{ userId: await funWalletAuth.getAddress() }],
uniqueId: await funWalletAuth.getWalletUniqueId()
})

const privateKey = generatePrivateKey()
const sponsor = new Auth({ privateKey })
const sponsorAddress = await sponsor.getAddress()

await configureEnvironment({
chain: "goerli",
apiKey: "ZrhepzWGxm74D0sqstuhT6dGrJxhoy8SZIToX6I3",
gasSponsor: {
sponsorAddress,
token: "0x0a1F0598A561af6b84A726bE007f581E812C3CDD",
usePermit: true
}
})

const tokenSponsor = new TokenSponsor()

const depositAmount = 0.005
const depositorAddress = sponsorAddress
const depositEth = await tokenSponsor.stake(depositorAddress, sponsorAddress, depositAmount)

const whitelistMode = await tokenSponsor.setToWhitelistMode(5, sponsorAddress)
const whitelistToken = await tokenSponsor.batchWhitelistTokens(sponsorAddress, ['0x0a1F0598A561af6b84A726bE007f581E812C3CDD'], [true])
const whitelistUser = await tokenSponsor.addSpenderToWhiteList(5, sponsorAddress, await funWallet.getAddress())
await sponsor.sendTxs([depositEth, whitelistToken, whitelistMode, whitelistUser])

const transferTokensToWallet = await new Token("0x0a1F0598A561af6b84A726bE007f581E812C3CDD").transfer(await funWallet.getAddress(), 100)
await sponsor.sendTxs([transferTokensToWallet])

const op = await funWallet.create(funWalletAuth, await sponsor.getAddress())
console.log(await funWallet.executeOperation(funWalletAuth, op))
}
createTokenSponsor()