Calling a smart contract function

When you want to call a function that is not a view function (meaning it changes the state of the blockchain), you need to send a transaction to the smart contract. This is done by using the sendTransaction method of MiniKit .

Installing MiniKit

We need to install the @worldcoin/minikit-js package to use MiniKit in our Mini App project so we can interact with smart contracts.


npm install @worldcoin/minikit-js


pnpm add @worldcoin/minikit-js


yarn add @worldcoin/minikit-js


bun add @worldcoin/minikit-js

Also make your life easier by installing viem . It comes with a lot of useful functions to interact with smart contracts.


npm install viem


pnpm add viem


yarn add viem


bun add viem

This libraries come installed by default when using the create-world-app command used in the Writing your first Mini App tutorial.

Here is a quick example to get you started

App.tsx


"use client"
 
import { MiniKit } from "@worldcoin/minikit-js"
import { erc20Abi } from "viem" // We import the ERC20 ABI from viem
 
const sendTransaction = async () => {
  // The following method will try to transfer
  // 1 WLD token from the World App to another address
  // using the `transfer` function of the ERC20 contract.
 
  const WLD_TOKEN_ADDRESS = "0x2cFca85d8E48F8EAB294be644d9E25C3030863003"
  const RECIPIENT_ADDRESS = "0x05a700132Fb88D4F565453153E6b05F2049fCb45"
 
  const { commandPayload, finalPayload } =
    await MiniKit.commandsAsync.sendTransaction({
      transaction: [
        {
          address: WLD_TOKEN_ADDRESS,
          abi: erc20Abi,
          functionName: "transfer",
          args: [
            RECIPIENT_ADDRESS, // Address you want to send tokens to
            parseEther("1"), // 1 WLD token
          ],
        },
      ],
    })
}

Here we are introduced to a some new concepts:

ABI: Application Binary Interface. This is essentially a guide or an interface that tells developers how to interact with smart contracts. An API if you will, but for smart contracts.
ERC20: An standard interface for tokens on the Ethereum blockchain . It defines how tokens can be transferred, how to get the balance of an account, and other basic functionalities.
parseEther: A utility function in viem that converts a number to its representation with 18 decimal places. This is necessary because smart contracts often use integers not decimals, that’s why we use a mantissa to represent the decimal places. For example, 1 WLD token is represented as 1000000000000000000 in the smart contract.