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WTF Ethers: 13. Encode Calldata

I've been revisiting ethers.js recently to refresh my understanding of the details and to write a simple tutorial called "WTF Ethers" for beginners.

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All the code and tutorials are open-sourced on GitHub:

In this lesson, we will introduce the interface class in ethers.js and use it to encode calldata.

Interface Class

The interface class in ethers.js abstracts the ABI encoding and decoding needed for interacting with contracts on the Ethereum network. ABI (Application Binary Interface) is a format similar to API, used to encode various types of data that contracts can process, enabling them to interact. For more information, see WTF Solidity 27: ABI encoding.

We can generate the interface variable from the abi, or directly retrieve it from the contract:

// Generated using abi
const interface = ethers.Interface(abi)
// Retrieved from the contract
const interface2 = contract.interface

The interface class encapsulates several encoding and decoding methods. When interacting with some special contracts (such as proxy contracts), you need to encode parameters and decode return values:

Note: The relevant function must be included in the abi.

  • getSighash(): Get the function selector, with the function name or function signature as the parameter.

    // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef'
  • encodeDeploy(): Encode the parameters of the constructor, which can then be appended to the contract bytecode.

    interface.encodeDeploy("Wrapped ETH", "WETH");
  • encodeFunctionData(): Encode the calldata of the function.

    interface.encodeFunctionData("balanceOf", ["0xc778417e063141139fce010982780140aa0cd5ab"]);
  • decodeFunctionResult(): Decode the return value of the function.

    interface.decodeFunctionResult("balanceOf", resultData)

Example: Interacting with the WETH contract on test network

Here, we will use the interface class to encode calldata and repeat the example of interacting with the test network WETH contract in Lesson 5.

  1. Create the provider and wallet variables.

    // Prepare Alchemy API, you can refer to 
    const ALCHEMY_GOERLI_URL = '';
    const provider = new ethers.JsonRpcProvider(ALCHEMY_GOERLI_URL);

    // Create the wallet object using the private key and provider
    const privateKey = '0x227dbb8586117d55284e26620bc76534dfbd2394be34cf4a09cb775d593b6f2b'
    const wallet = new ethers.Wallet(privateKey, provider)
  2. Create the WETH contract instance

    // WETH ABI
    const abiWETH = [
    "function balanceOf(address) public view returns(uint)",
    "function deposit() public payable",
    // WETH contract address (Goerli test network)
    const addressWETH = '0xb4fbf271143f4fbf7b91a5ded31805e42b2208d6'
    // Declare the WETH contract
    const contractWETH = new ethers.Contract(addressWETH, abiWETH, wallet)
  3. Call the balanceOf() function to read the WETH balance of the wallet address.

    const address = await wallet.getAddress()
    // 1. Read the on-chain information of the WETH contract (WETH ABI)
    console.log("\n1. Read WETH balance")
    // Encode calldata
    const param1 = contractWETH.interface.encodeFunctionData(
    console.log(`Encoded result: ${param1}`)
    // Create transaction
    const tx1 = {
    to: addressWETH,
    data: param1
    // Send transaction, view/pure operations can use
    const balanceWETH = await
    console.log(`WETH balance before deposit: ${ethers.formatEther(balanceWETH)}\n`)

    Check WETH balance

  4. Call the deposit() function to convert 0.001 ETH to 0.001 WETH, and print the transaction details and balance. You can see the balance change.

    // Encode calldata
    const param2 = contractWETH.interface.encodeFunctionData(
    console.log(`Encoded result: ${param2}`)
    // Create transaction
    const tx2 = {
    to: addressWETH,
    data: param2,
    value: ethers.parseEther("0.001")}
    // Send transaction, write operations require wallet.sendTransaction(tx)
    const receipt1 = await wallet.sendTransaction(tx2)
    // Wait for the transaction to be confirmed
    await receipt1.wait()
    console.log(`Transaction details:`)
    const balanceWETH_deposit = await contractWETH.balanceOf(address)
    console.log(`WETH balance after deposit: ${ethers.formatEther(balanceWETH_deposit)}\n`)

    Calling the deposit() function


In this lesson, we introduced the interface classes in ethers.js and used them to encode calldata and interact with the WETH contract. When interacting with specific contracts (such as proxy contracts), you may need to use these methods to encode the parameters and decode the return values.