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Loop Decoder

How to decode a Calldata

In this guide, we will go through the process of decoding EVM Calldata using Loop Decoder. As an example we will decode a Calldata for Gnosis Safe Multisend transaction that has deep nested calls.

We recomend to copy all snipepts to a typescript project and run it at the end of this guide, or or you can copy the whole example from this file: Full Example Code. Do not forget to replace the placeholder YourApiKeyToken with your own free Etherscan API key.

Prerequisites

Create a new project

Optionally, you can create a new project to follow along, or skip to Required packages.

  1. Install Bun: First, make sure you have Bun installed on your system. If you haven’t installed it yet, you can do so using npm:
Terminal window
npm install -g bun
  1. Generate and initialize a new project:
Terminal window
mkdir example-decode && cd example-decode
bun init

Required packages

For this guide, you will need to have the following packages installed:

Terminal window
bun install @3loop/transaction-decoder viem

Data Sources

Loop Decoder requires some data sources to be able to decode transactions. We will need an RPC provider, a data source to fetch Contracts ABIs and a data source to fetch contract meta-information, such as token name, decimals, symbol, etc.

RPC Provider

We will start by creating a function which will return an object with PublicClient based on the chain ID. For the sake of this example, we will only support mainnet.

index.ts
import { createPublicClient, http } from 'viem'


// Create a public client for the Ethereum Mainnet network
const getPublicClient = (chainId: number) => {
  return {
    client: createPublicClient({
      transport: http('https://rpc.ankr.com/eth'),
    }),
  }
}

ABI loader

To avoid making unecessary calls to third-party APIs, Loop Decoder uses an API that allows cache. For this example, we will keep it simple and use an in-memory cache. We will also use some strategies to download contract ABIs from Etherscan and 4byte.directory. You can find more information about the strategies in the Strategies reference.

Create a cache for contract ABI and add your free Etherscan API key instead of the placeholder YourApiKeyToken:

index.ts
import {
  EtherscanStrategyResolver,
  FourByteStrategyResolver,
  VanillaAbiStore,
  ContractABI,
} from '@3loop/transaction-decoder'


// Create an in-memory cache for the ABIs
const abiCache = new Map<string, ContractABI>()


const abiStore: VanillaAbiStore = {
  // Define the strategies to use for fetching the ABIs
  strategies: [
    EtherscanStrategyResolver({
      apikey: 'YourApiKeyToken',
    }),
    FourByteStrategyResolver(),
  ],


  // Get the ABI from the cache
  // Get it by contract address, event name or signature hash
  get: async ({ address, event, signature }) => {
    const value = abiCache.get(address)
    if (value) {
      return {
        status: 'success',
        result: value,
      }
    } else if (event) {
      const value = abiCache.get(event)
      if (value) {
        return {
          status: 'success',
          result: value,
        }
      }
    } else if (signature) {
      const value = abiCache.get(signature)
      if (value) {
        return {
          status: 'success',
          result: value,
        }
      }
    }


    return {
      status: 'empty',
      result: null,
    }
  },


  // Set the ABI in the cache
  // Store it by contract address, event name or signature hash
  set: async (_key, value) => {
    if (value.status === 'success') {
      if (value.result.type === 'address') {
        abiCache.set(value.result.address, value.result)
      } else if (value.result.type === 'event') {
        abiCache.set(value.result.event, value.result)
      } else if (value.result.type === 'func') {
        abiCache.set(value.result.signature, value.result)
      }
    }
  },
}

Contract Metadata loader

Create an in-memory cache for contract meta-information. Using ERC20RPCStrategyResolver we will automatically retrieve token meta information from the contract such as token name, decimals, symbol, etc.

index.ts
import type { ContractData, VanillaContractMetaStore } from '@3loop/transaction-decoder'
import { ERC20RPCStrategyResolver } from '@3loop/transaction-decoder'


// Create an in-memory cache for the contract meta-information
const contractMetaCache = new Map<string, ContractData>()


const contractMetaStore: VanillaContractMetaStore = {
  // Define the strategies to use for fetching the contract meta-information
  strategies: [ERC20RPCStrategyResolver],


  // Get the contract meta-information from the cache
  get: async ({ address, chainID }) => {
    const key = `${address}-${chainID}`.toLowerCase()
    const value = contractMetaCache.get(key)


    if (value) {
      return {
        status: 'success',
        result: value,
      }
    }


    return {
      status: 'empty',
      result: null,
    }
  },


  // Set the contract meta-information in the cache
  set: async ({ address, chainID }, result) => {
    const key = `${address}-${chainID}`.toLowerCase()


    if (result.status === 'success') {
      contractMetaCache.set(key, result.result)
    }
  },
}

Finally, you can create a new instance of the LoopDecoder class:

import { TransactionDecoder } from '@3loop/transaction-decoder'


const decoder = new TransactionDecoder({
  getPublicClient: getPublicClient,
  abiStore: abiStore,
  contractMetaStore: contractMetaStore,
})

Decoding a Calldata

Now that we have all the necessary components, we can start decoding a Calldata. For this example, we will use the following Calldata:

async function main() {
  try {
    const decoded = await decoder.decodeCalldata({
      data: '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',
      chainID: 1,
      contractAddress: '0x40A2aCCbd92BCA938b02010E17A5b8929b49130D',
    })


    console.log(JSON.stringify(decoded, null, 2))
  } catch (e) {
    console.error(JSON.stringify(e, null, 2))
  }
}


main()

Run the script:

Terminal window
bun run index.ts

Expected output:

{
  "name": "multiSend",
  "signature": "multiSend(bytes)",
  "type": "function",
  "params": [
    {
      "name": "transactions",
      "type": "bytes",
      "value": "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",
      "valueDecoded": {
        "name": "transactions",
        "signature": "transactions((uint256,address,uint256,uint256,bytes)[])",
        "type": "function",
        "params": [
          {
            "name": "unknown",
            "type": "tuple",
            "components": [
              {
                "name": "unknown",
                "type": "tuple",
                "components": [
                  {
                    "name": "operation",
                    "type": "uint256",
                    "value": "0"
                  },
                  {
                    "name": "to",
                    "type": "address",
                    "value": "0x2D8880BcC0618DBCC5d516640015A69e28fdC406"
                  },
                  {
                    "name": "value",
                    "type": "uint256",
                    "value": "0"
                  },
                  {
                    "name": "dataLength",
                    "type": "uint256",
                    "value": "1544"
                  },
                  //...