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Bridging ERC-20 Tokens With the Optimism SDK

Bridging ERC-20 Tokens to OP Mainnet With the Optimism SDK

This tutorial explains how you can use the Optimism SDK (opens in a new tab) to bridge ERC-20 tokens from L1 (Ethereum or Sepolia) to L2 (OP Mainnet or OP Sepolia). The Optimism SDK is an easy way to add bridging functionality to your javascript-based application. It also provides some safety rails to prevent common mistakes that could cause tokens to be made inaccessible.

Behind the scenes, the Optimism SDK uses the Standard Bridge contracts to transfer tokens. Make sure to check out the Standard Bridge guide if you want to learn more about how the bridge works under the hood.

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The Standard Bridge does not support fee on transfer tokens (opens in a new tab) or rebasing tokens (opens in a new tab) because they can cause bridge accounting errors.

Supported Networks

The Optimism SDK supports any of the Superchain networks. Some Superchain networks (opens in a new tab) are already included in the SDK by default. If you want to use a network that isn't included by default you can simply instantiate the SDK with the appropriate contract addresses.

Dependencies

Create a Demo Project

You're going to use the Optimism SDK for this tutorial. Since the Optimism SDK is a Node.js (opens in a new tab) library, you'll need to create a Node.js project to use it.

Make a Project Folder

mkdir op-sample-project
cd op-sample-project

Initialize the Project

pnpm init

Install the Optimism SDK

pnpm add @eth-optimism/sdk

Install ethers.js

pnpm add ethers@^5

Want to create a new wallet for this tutorial? If you have cast (opens in a new tab) installed you can run cast wallet new in your terminal to create a new wallet and get the private key.

Get ETH on Sepolia and OP Sepolia

This tutorial explains how to bridge tokens from Sepolia to OP Sepolia. You will need to get some ETH on both of these testnets.

You can use this faucet (opens in a new tab) to get ETH on Sepolia. You can use the Superchain Faucet (opens in a new tab) to get ETH on OP Sepolia.

Add a Private Key to Your Environment

You need a private key in order to sign transactions. Set your private key as an environment variable with the export command. Make sure this private key corresponds to an address that has ETH on both Sepolia and OP Sepolia.

export TUTORIAL_PRIVATE_KEY=0x...

Start the Node REPL

You're going to use the Node REPL to interact with the Optimism SDK. To start the Node REPL run the following command in your terminal:

node

This will bring up a Node REPL prompt that allows you to run javascript code.

Import Dependencies

You need to import some dependencies into your Node REPL session.

Import the Optimism SDK

const optimism = require("@eth-optimism/sdk")

Import ethers.js

const ethers = require("ethers")

Set Session Variables

You'll need a few variables throughout this tutorial. Let's set those up now.

Load your private key

const privateKey = process.env.TUTORIAL_PRIVATE_KEY

Create the RPC providers and wallets

const l1Provider = new ethers.providers.StaticJsonRpcProvider("https://rpc.ankr.com/eth_sepolia")
const l2Provider = new ethers.providers.StaticJsonRpcProvider("https://sepolia.optimism.io")
const l1Wallet = new ethers.Wallet(privateKey, l1Provider)
const l2Wallet = new ethers.Wallet(privateKey, l2Provider)

Set the L1 and L2 ERC-20 addresses

This tutorial uses existing ERC-20 tokens that have been deployed on Sepolia and OP Sepolia. These tokens are designed for testing the bridging process.

const l1Token = "0x5589BB8228C07c4e15558875fAf2B859f678d129"
const l2Token = "0xD08a2917653d4E460893203471f0000826fb4034"
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If you're coming from the Bridging Your Standard ERC-20 Token to OP Mainnet Using the Standard Bridge or Bridging Your Custom ERC-20 Token to OP Mainnet Using the Standard Bridge tutorials, you can use the addresses of your own ERC-20 tokens here instead.

Get L1 Tokens

You're going to need some tokens on L1 that you can bridge to L2. The L1 testing token located at 0x5589BB8228C07c4e15558875fAf2B859f678d129 (opens in a new tab) has a faucet function that makes it easy to get tokens.

Set the ERC20 ABI

const erc20ABI = [{ constant: true, inputs: [{ name: "_owner", type: "address" }], name: "balanceOf", outputs: [{ name: "balance", type: "uint256" }], type: "function" }, { inputs: [], name: "faucet", outputs: [], stateMutability: "nonpayable", type: "function" }]

Create a Contract instance for the L1 token

const l1ERC20 = new ethers.Contract(l1Token, erc20ABI, l1Wallet)

Request some tokens

tx = await l1ERC20.faucet()
await tx.wait()

Check your token balance

console.log((await l1ERC20.balanceOf(l1Wallet.address)).toString())

Deposit Tokens

Now that you have some tokens on L1 you can deposit those tokens into the L1StandardBridge contract. You'll then receive the same number of tokens on L2 in return.

Define the amount to deposit

The testing token has 18 decimal places, so you'll want to define a variable that represents one token.

const oneToken = 1000000000000000000n

Create a CrossChainMessenger instance

The Optimism SDK exports a CrossChainMessenger class that makes it easy to interact with the L1StandardBridge contract.

Create an instance of the CrossChainMessenger class:

const messenger = new optimism.CrossChainMessenger({
  l1ChainId: 11155111, // 11155111 for Sepolia, 1 for Ethereum
  l2ChainId: 11155420, // 11155420 for OP Sepolia, 10 for OP Mainnet
  l1SignerOrProvider: l1Wallet,
  l2SignerOrProvider: l2Wallet,
})

Allow the Standard Bridge to access your tokens

Before you can deposit your tokens, you'll need to give the Standard Bridge contract an allowance of tokens on L1. This will allow the Standard Bridge to pull these tokens out of your address and escrow inside the bridge.

tx = await messenger.approveERC20(l1Token, l2Token, oneToken)
await tx.wait()

Deposit your tokens

Now you can deposit your tokens into the Standard Bridge contract.

tx = await messenger.depositERC20(l1Token, l2Token, oneToken)
await tx.wait()
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Using a smart contract wallet? As a safety measure, depositERC20 will fail if you try to deposit ETH from a smart contract wallet without specifying a recipient. Add the recipient option to the depositERC20 call to fix this. Check out the Optimism SDK docs (opens in a new tab) for more info on the options you can pass to depositERC20.

Wait for the deposit to be relayed

You can use the waitForMessageStatus function to wait for the deposit to be relayed to L2.

await messenger.waitForMessageStatus(tx.hash, optimism.MessageStatus.RELAYED)

Check your token balance on L1

You should now have one less token on L1.

console.log((await l1ERC20.balanceOf(l1Wallet.address)).toString())

Create a Contract instance for the L2 token

const l2ERC20 = new ethers.Contract(l2Token, erc20ABI, l2Wallet)

Check your token balance on L2

You should now have one more token on L2.

console.log((await l2ERC20.balanceOf(l2Wallet.address)).toString())

Withdraw Tokens

You just bridged some tokens from L1 to L2. Nice! Now you're going to repeat the process in reverse to bridge some tokens from L2 to L1.

Start your withdrawal on L2

The first step to withdrawing tokens from L2 to L1 is to start the withdrawal on L2.

const withdrawal = await messenger.withdrawERC20(l1Token, l2Token, oneToken)
await withdrawal.wait()

Check your token balance on L2

You should now have one less token on L2, but your token balance on L1 will not have changed yet.

console.log((await l2ERC20.balanceOf(l2Wallet.address)).toString())

Wait until the withdrawal is ready to prove

The second step to withdrawing tokens from L2 to L1 is to prove to the bridge on L1 that the withdrawal happened on L2. You first need to wait until the withdrawal is ready to prove.

await messenger.waitForMessageStatus(withdrawal.hash, optimism.MessageStatus.READY_TO_PROVE)

This step can take a few minutes. Feel free to take a quick break while you wait.

Prove the withdrawal on L1

Once the withdrawal is ready to be proven, you'll send an L1 transaction to prove that the withdrawal happened on L2.

await messenger.proveMessage(withdrawal.hash)

Wait until the withdrawal is ready for relay

The final step to withdrawing tokens from L2 to L1 is to relay the withdrawal on L1. This can only happen after the fault proof period has elapsed. On OP Sepolia, this is only a few seconds. On OP Mainnet, this takes 7 days.

await messenger.waitForMessageStatus(withdrawal.hash, optimism.MessageStatus.READY_FOR_RELAY)

Relay the withdrawal on L1

Once the withdrawal is ready to be relayed you can finally complete the withdrawal process.

await messenger.finalizeMessage(withdrawal.hash)

Wait until the withdrawal is relayed

Now you simply wait until the message is relayed.

await messenger.waitForMessageStatus(withdrawal.hash, optimism.MessageStatus.RELAYED)

Check your token balance on L1

You should now have one more token on L1.

console.log((await l1ERC20.balanceOf(l1Wallet.address)).toString())

Next Steps

Congrats! You've just deposited and withdrawn tokens using the Optimism SDK. You should now be able to write applications that use the Optimism SDK to transfer ERC-20 tokens between L1 and L2. Although this tutorial used Sepolia and OP Sepolia, the same process works for Ethereum and OP Mainnet.