***

title: How to Generate Calldata for NFT Checkout
slug: guides/how-to-generate-calldata-for-nft-checkout
subtitle: Generate the smart contract call data required for Transak NFT Checkout
---------------------------------------------------------------------------------

## What is calldata?

`Calldata` is the encoded hex data for the smart contract function that Transak will execute during NFT Checkout.

It tells the contract:

* which function to call
* which arguments to pass
* the exact order and format of those arguments

For NFT Checkout, you generate this calldata from your contract ABI and function inputs, then pass it to Transak as the `calldata` parameter.

## Prerequisite: Partner setup before you start

Before you generate calldata, make sure you already have:

* The ABI for the contract function that should be executed
* The exact `function name`
* The exact `arguments in the correct order`
* The Transak NFT Checkout smart contract address for the target chain if your function accepts a recipient wallet address such as `to`, `recipient`, or `buyer`

<Frame>
  <div>
    If you use a proxy contract, encode the calldata with the ABI of the **implementation contract**, `not the proxy`.
  </div>
</Frame>

<Frame>
  <div>
    If your function accepts a wallet parameter such as `to`, `recipient`, or `buyer`, pass Transak's NFT Checkout smart contract address for that chain instead of the end user's wallet address.
  </div>
</Frame>

Use [NFT Checkout smart contract addresses](/products/nft-checkout#transak-smart-contract-addresses) to get the correct chain-specific address.

## How to generate calldata

<Steps>
  <Step title="Declare the ABI">
    ```javascript
    const getSupplyCalldata = (contributor: string, amount: number | null) => {
      if (!amount) return;

      /**
       * ABI for Supply function on AAVE SmartContract
       * You can find ABI of AAVE contract here:
       * https://mumbai.polygonscan.com/address/0xbadd48c3eb42a10db791d7b02e3c07fbf95b3155#code
       */
      let ABI = [
        {
          inputs: [
            {
              internalType: "contract IPoolAddressesProvider",
              name: "provider",
              type: "address",
            },
          ],
          stateMutability: "nonpayable",
          type: "constructor",
        },
        {
          anonymous: false,
          inputs: [
            {
              indexed: true,
              internalType: "address",
              name: "reserve",
              type: "address",
            },
            {
              indexed: true,
              internalType: "address",
              name: "backer",
              type: "address",
            },
            {
              indexed: false,
              internalType: "uint256",
              name: "amount",
              type: "uint256",
            },
            {
              indexed: false,
              internalType: "uint256",
              name: "fee",
              type: "uint256",
            },
          ],
          name: "BackUnbacked",
          type: "event",
        },
        {
          anonymous: false,
          inputs: [
            {
              indexed: true,
              internalType: "address",
              name: "reserve",
              type: "address",
            },
            {
              indexed: false,
              internalType: "address",
              name: "user",
              type: "address",
            },
            // ABI shortened here for practical purposes
          ],
          name: "BalanceTransfer",
          type: "event",
        },
      ];
    };
    ```
  </Step>

  <Step title="Contructing the calldata">
    ```typescript
    let parsedAmmount = parseUnits(amount.toString(), 8);
    // Constructing the CallData
    return new Interface(ABI).encodeFunctionData("supply", [
      "0x2Fa2e7a6dEB7bb51B625336DBe1dA23511914a8A", // asset contract address for WBTC as an example on mumbai testnet
      parsedAmmount, // amount user wants to buy
      contributor, // address of the user who is depositing the funds
      0, // referal code. Will be 0 in case of aave
    ]);
    ```
  </Step>

  <Step title="Get the calldata">
    ```typescript
    export default function App() {
      useEffect(() => {
        const depositAmount = 0.0008; // amount user want to deposit to protocol

        const calldata = getSupplyCalldata(
          USER_WALLET_ADDRESS, // user wallet address
          depositAmount // amount user want to deposit. Transak will convert it to the fiat equivalent and display it to the user
        );
      }
    }
    ```
  </Step>

  <Step title="Pass the calldata to Transak">
    If you are passing calldata through a widget URL or through `createWidgetUrl`, pass the raw hex string directly as the `calldata` value.

    <CodeBlocks>
      ```bash title="Create Widget URL"
      curl --request POST \
       --url https://api-gateway-stg.transak.com/api/v2/auth/session \
       --header 'accept: application/json' \
       --header 'access-token: YOUR_ACCESS_TOKEN' \
       --header 'content-type: application/json' \
       --data '{
      "widgetParams": { 
        "apiKey": "API_KEY", 
        "walletAddress": "USER_WALLET_ADDRESS", 
        "isNFT": true, 
        "calldata": "0xsdad....", 
        "nftData": [
          {
            imageURL: "https://pokemon-nfts.s3.ap-southeast-2.amazonaws.com/images/1.png", // string
            nftName: "Pokemon Metadata Legends",                                            // string
            collectionAddress: "0x8a20e9e8e736643161ce6a2fe8dd8dd62050cd1e",               // string
            tokenID: ["6", "7", "8"],                                                      // string[]
            price: [15, 15, 15],                                                           // number[]
            quantity: 3,                                                                   // number
            nftType: "ERC721"                                                              // "ERC721" | "ERC1155"
          }
        ],
        "contractId": "6965438baf941bf38cd", 
      }
      ```
    </CodeBlocks>

    If you use the [SDK](/integration/web/js-sdk), pass the raw calldata directly in the SDK configuration.
  </Step>
</Steps>

## Full Example To Generate Calldata

```javascript
const getSupplyCalldata = (contributor: string, amount: number | null) => {
  if (!amount) return;

  /**
   * ABI for Supply function on AAVE SmartContract
   * You can find ABI of AAVE contract here:
   * https://mumbai.polygonscan.com/address/0xbadd48c3eb42a10db791d7b02e3c07fbf95b3155#code
   */
  let ABI = [
    {
      inputs: [
        {
          internalType: "contract IPoolAddressesProvider",
          name: "provider",
          type: "address",
        },
      ],
      stateMutability: "nonpayable",
      type: "constructor",
    },
    {
      anonymous: false,
      inputs: [
        {
          indexed: true,
          internalType: "address",
          name: "reserve",
          type: "address",
        },
        {
          indexed: true,
          internalType: "address",
          name: "backer",
          type: "address",
        },
        {
          indexed: false,
          internalType: "uint256",
          name: "amount",
          type: "uint256",
        },
        {
          indexed: false,
          internalType: "uint256",
          name: "fee",
          type: "uint256",
        },
      ],
      name: "BackUnbacked",
      type: "event",
    },
    {
      anonymous: false,
      inputs: [
        {
          indexed: true,
          internalType: "address",
          name: "reserve",
          type: "address",
        },
        {
          indexed: false,
          internalType: "address",
          name: "user",
          type: "address",
        },
        // ABI shortened here for practical purposes
      ],
      name: "BalanceTransfer",
      type: "event",
    },
  ];

  let parsedAmount = parseUnits(amount.toString(), 8);

  return new Interface(ABI).encodeFunctionData("supply", [
    "0x2Fa2e7a6dEB7bb51B625336DBe1dA23511914a8A", // asset contract address for WBTC on Mumbai testnet
    parsedAmount, // amount user wants to buy
    contributor, // address of the user who is depositing the funds
    0, // referral code. Will be 0 in case of AAVE
  ]);
};

// Create Widget URL API

const calldata = getSupplyCalldata(USER_WALLET_ADDRESS, 0.0008);

console.log("Use this in the SDK config:", calldata);
console.log(
  "Or pass it in the widget URL:",
  `"widgetParams": { calldata: ${calldata} }`
);
```