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Building a user interface for your contract

Ori Pomerantz
1 novembre 2023
15 minuti letti minute read

You found a feature we need in the Ethereum ecosystem. You wrote the smart contracts to implement it, and maybe even some related code that runs offchain. This is great! Unfortunately, without a user interface you aren't going to have any users, and the last time you wrote a web site people used dial-up modems and JavaScript was new.

This article is for you. I assume you know programming, and maybe a bit of JavaScript and HTML, but that your user interface skills are rusty and out of date. Together we will go over a simple modern application so you'll see how it's done these days.

Why is this important

In theory, you could just have people use Etherscan(opens in a new tab) or Blockscout(opens in a new tab) to interact with your contracts. That will be great for the experienced Ethereans. But we are trying to serve another billion people(opens in a new tab). This won't happen without a great user experience, and a friendly user interface is a big part of that.

Greeter application

There is a lot of theory behind for a modern UI works, and a lot of good sites(opens in a new tab) that explain it(opens in a new tab). Instead of repeating the fine work done by those sites, I'm going to assume you prefer to learn by doing and start with an application you can play with. You still need the theory to get things done, and we'll get to it - we'll just go source file by source file, and discuss things as we get to them.


  1. If necessary, add the Holesky blockchain(opens in a new tab) to your wallet and get test ETH(opens in a new tab).

  2. Clone the github repository.

    1git clone
  3. Install the necessary packages.

    1cd 20230801-modern-ui
    2pnpm install
  4. Start the application.

    1pnpm dev
  5. Browse to the URL shown by the application. In most cases, that is http://localhost:5173/(opens in a new tab).

  6. You can see the contract source code, a slightly modified version of Hardhat's Greeter, on a blockchain explorer(opens in a new tab).

File walk through


This file is standard HTML boilerplate except for this line, which imports the script file.

1<script type="module" src="/src/main.tsx"></script>


The file extension tells us that this file is a React component(opens in a new tab) written in TypeScript(opens in a new tab), an extension of JavaScript that supports type checking(opens in a new tab). TypeScript is compiled into JavaScript, so we can use it for client-side execution.

1import '@rainbow-me/rainbowkit/styles.css'
2import { RainbowKitProvider } from '@rainbow-me/rainbowkit'
3import * as React from 'react'
4import * as ReactDOM from 'react-dom/client'
5import { WagmiConfig } from 'wagmi'
6import { chains, config } from './wagmi'

Import the library code we need.

1import { App } from './App'

Import the React component that implements the application (see below).


Create the root React component. The parameter to render is JSX(opens in a new tab), an extension language that uses both HTML and JavaScript/TypeScript. The exclamation point here tells the TypeScript component: "you don't know that document.getElementById('root') will be a valid parameter to ReactDOM.createRoot, but don't worry - I'm the developer and I'm telling you there will be".

1 <React.StrictMode>

The application is going inside a React.StrictMode component(opens in a new tab). This component tells the React library to insert additional debugging checks, which is useful during development.

1 <WagmiConfig config={config}>

The application is also inside a WagmiConfig component(opens in a new tab). The wagmi (we are going to make it) library(opens in a new tab) connects the React UI definitions with the viem library(opens in a new tab) for writing an Ethereum decentralized application.

1 <RainbowKitProvider chains={chains}>

And finally, a RainbowKitProvider component(opens in a new tab). This component handles logging on and the communication between the wallet and the application.

1 <App />

Now we can have the component for the application, which actually implements the UI. The /> at the end of the component tells React that this component doesn't have any definitions inside it, as per the XML standard.

1 </RainbowKitProvider>
2 </WagmiConfig>
3 </React.StrictMode>,

Of course, we have to close off the other components.


1import { ConnectButton } from '@rainbow-me/rainbowkit'
2import { useAccount } from 'wagmi'
3import { Greeter } from './components/Greeter'
5export function App() {

This is the standard way to create a React component - define a function that is called every time it needs to be rendered. This function typically has some TypeScript or JavaScript code on top, followed by a return statement that returns the JSX code.

1 const { isConnected } = useAccount()

Here we use useAccount(opens in a new tab) to check if we are connected to a blockchain through a wallet or not.

By convention, in React functions called use... are hooks(opens in a new tab) that return some kind of data. When you use such hooks, not only does your component get the data, but when that data changes the component is re-rendered with the updated information.

1 return (
2 <>

The JSX of a React component has to return one component. When we have multiple components and we don't have anything that wraps up "naturally" we use an empty component (<> ... </>) to make them into a single component.

1 <h1>Greeter</h1>
2 <ConnectButton />

We get the ConnectButton component(opens in a new tab) from RainbowKit. When we are not connected, it gives us a Connect Wallet button that opens a modal that explains wallets and lets you choose which one you use. When we are connected, it displays the blockchain we use, our account address, and our ETH balance. We can use these displays to switch network or to disconnect.

1 {isConnected && (

When we need to insert actual JavaScript (or TypeScript that will be compiled to JavaScript) into a JSX, we use brackets ({}).

The syntax a && b is short for a ? b : a(opens in a new tab). That is, if a is true it evaluates to b and otherwise it evaluates to a (which can be false, 0, etc). This is an easy way to tell React that a component should only be displayed if a certain condition is fulfilled.

In this case, we only want to show the user Greeter if the user is connected to a blockchain.

1 <Greeter />
2 )}
3 </>
4 )


This file contains most of the UI functionality. It includes definitions that would normally be in multiple files, but as this is a tutorial the program is optimized for being easy to understand the first time, rather than performance or ease of maintenance.

1import { useState, ChangeEventHandler } from 'react'
2import { useNetwork,
3 useContractRead,
4 usePrepareContractWrite,
5 useContractWrite,
6 useContractEvent
7 } from 'wagmi'

We use these library functions. Again, they are explained below where they are used.

1import { AddressType } from 'abitype'

The abitype library(opens in a new tab) provides us with TypeScript definitions for various Ethereum data types, such as AddressType(opens in a new tab).

1let greeterABI = [
2 .
3 .
4 .
5] as const // greeterABI

The ABI for the Greeter contract. If you are developing the contracts and UI at the same time you'd normally put them in the same repository and use the ABI generated by the Solidity compiler as a file in your application. However, this is not necessary here because the contract is already developed and not going to change.

1type AddressPerBlockchainType = {
2 [key: number]: AddressType

TypeScript is strongly typed. We use this definition to specify the address in which the Greeter contract is deployed on different chains. The key is a number (the chainId), and the value is an AddressType (an address).

1const contractAddrs: AddressPerBlockchainType = {
2 // Holesky
3 17000: '0x432d810484AdD7454ddb3b5311f0Ac2E95CeceA8',
5 // Sepolia
6 11155111: '0x7143d5c190F048C8d19fe325b748b081903E3BF0'

The address of the contract on the two supported networks: Holesky(opens in a new tab) and Sepolia(opens in a new tab).

Note: There is actually a third definition, for Redstone Holesky, it will be explained below.

1type ShowObjectAttrsType = {
2 name: string,
3 object: any

This type is used as a parameter to the ShowObject component (explained later). It includes the name of the object and its value, which are displayed for debugging purposes.

1type ShowGreetingAttrsType = {
2 greeting: string | undefined

At any moment in time we may either know what the greeting is (because we read it from the blockchain) or not know (because we haven't received it yet). So it is useful to have a type that can be either a string or nothing.

1const Greeter = () => {

Finally, we get the define the component.

1 const { chain } = useNetwork()

Information about the chain we are using, courtesy of wagmi(opens in a new tab). Because this is a hook (use...), every time this information changes the component gets redrawn.

1 const greeterAddr = chain && contractAddrs[]

The address of the Greeter contract, which varies by chain (and which is undefined if we don't have chain information or we are on a chain without that contract).

1 const readResults = useContractRead({
2 address: greeterAddr,
3 abi: greeterABI,
4 functionName: "greet" , // No arguments
5 watch: true
6 })

The useContractRead hook(opens in a new tab) reads information from a contract. You can see exactly what information it returns expand readResults in the UI. In this case we want it to keep looking so we'll be informed when the greeting changes.

Note: We could listen to setGreeting events(opens in a new tab) to know when the greeting changes and update that way. However, while it may be more efficient, it will not apply in all cases. When the user switches to a different chain the greeting also changes, but that change is not accompanied by an event. We could have one part of the code listening for events and another to identify chain changes, but that would be more complicated than just setting the watch parameter(opens in a new tab).

1 const [ newGreeting, setNewGreeting ] = useState("")

React's useState hook(opens in a new tab) lets us specify a state variable, whose value persists from one rendering of the component to another. The initial value is the parameter, in this case the empty string.

The useState hook returns a list with two values:

  1. The current value of the state variable.
  2. A function to modify the state variable when needed. As this is a hook, every time it is called the component is rendered again.

In this case, we are using a state variable for the new greeting the user wants to set.

1 const greetingChange : ChangeEventHandler<HTMLInputElement> = (evt) =>
2 setNewGreeting(

This is the event handler for when the new greeting input field changes. The type, ChangeEventHandler<HTMLInputElement>(opens in a new tab), specifies that this is handler for a value change of an HTML input element. The <HTMLInputElement> part is used because this is a generic type(opens in a new tab).

1 const preparedTx = usePrepareContractWrite({
2 address: greeterAddr,
3 abi: greeterABI,
4 functionName: 'setGreeting',
5 args: [ newGreeting ]
6 })
7 const workingTx = useContractWrite(preparedTx.config)

This is the process to submit a blockchain transaction from the client perspective:

  1. Send the transaction to a node in the blockchain using eth_estimateGas(opens in a new tab).
  2. Wait for a response from the node.
  3. When the response is received, ask the user to sign the transaction through the wallet. This step has to happen after the node response is received because the user is shown the gas cost of the transaction before signing it.
  4. Wait for the user for approve.
  5. Send the transaction again, this time using eth_sendRawTransaction(opens in a new tab).

Step 2 is likely to take a perceptible amount of time, during which users would wonder if their command was really received by the user interface and why they aren't being asked to sign the transaction already. That makes for bad user experience (UX).

The solution is to use prepare hooks(opens in a new tab). Every time that a parameter changes, immediately send the node the eth_estimateGas request. Then, when the user actually wants to send the transaction (in this case by pressing Update greeting), the gas cost is known and the user can see the wallet page immediately.

1 return (

Now we can finally create the actual HTML to return.

1 <>
2 <h2>Greeter</h2>
3 {
4 !readResults.isError && !readResults.isLoading &&
5 <ShowGreeting greeting={} />
6 }
7 <hr />

Create a ShowGreeting component (explained below), but only if the greeting was read successfully from the blockchain.

1 <input type="text"
2 value={newGreeting}
3 onChange={greetingChange}
4 />

This is the input text field where the user can set a new greeting. Every time the user presses a key, we call greetingChange which calls setNewGreeting. As setNewGreeting comes from the useState hook, it causes the Greeter component to be rendered again. This means that:

  • We need to specify value to keep the value of the new greeting, because otherwise it would turn back into the default, the empty string.
  • usePrepareContractWrite is called every time newGreeting changes, which means it is always going to have the latest newGreeting in the prepared transaction.
1 <button disabled={!workingTx.write}
2 onClick={workingTx.write}
3 >
4 Update greeting
5 </button>

If there is no workingTx.write then we are still waiting for information necessary for sending the greeting update, so the button is disabled. If there is a workingTx.write value then that is the function to call to send the transaction.

1 <hr />
2 <ShowObject name="readResults" object={readResults} />
3 <ShowObject name="preparedTx" object={preparedTx} />
4 <ShowObject name="workingTx" object={workingTx} />
5 </>
6 )

Finally, to help you see what we're doing, show the three objects we use:

  • readResults
  • preparedTx
  • workingTx

This component shows

1const ShowGreeting = (attrs : ShowGreetingAttrsType) => {

A component function receives a parameter with all the attributes of the component.

1 return <b>{attrs.greeting}</b>

For information purposes, we use the ShowObject component to show the important objects (readResults for reading the greeting and preparedTx and workingTx for transactions we create).

1const ShowObject = (attrs: ShowObjectAttrsType ) => {
2 const keys = Object.keys(attrs.object)
3 const funs = keys.filter(k => typeof attrs.object[k] == "function")
4 return <>
5 <details>

We don't want to clutter the UI with all the information, so to make it possible to view them or close them, we use a details(opens in a new tab) tag.

1 <summary>{}</summary>
2 <pre>
3 {JSON.stringify(attrs.object, null, 2)}

Most of the fields are displayed using JSON.stringify(opens in a new tab).

1 </pre>
2 { funs.length > 0 &&
3 <>
4 Functions:
5 <ul>

The exception is functions, which aren't part of the JSON standard(opens in a new tab), so they have to be displayed separately.

1 {, i) =>

Within JSX, code inside { curly brackets } is interpreted as JavaScript. Then, the code inside the ( regular brackets ), is interpreted again as JSX.

1 (<li key={i}>{f}</li>)
2 )}

React requires tags in the DOM Tree(opens in a new tab) to have distinct identifiers. This means that children of the same tag (in this case, the unordered list(opens in a new tab)), need different key attributes.

1 </ul>
2 </>
3 }
4 </details>
5 </>

End the various HTML tags.

The final export
1export { Greeter }

The Greeter component is the one we need to export for the application.


Finally, various definitions related to WAGMI are in src/wagmi.ts. I am not going to explain everything here, because most of it is boilerplate you are unlikely to need to change.

The code here isn't exactly the same as on github(opens in a new tab) because later in the article we add another chain (Redstone Holesky(opens in a new tab)).

1import { getDefaultWallets } from '@rainbow-me/rainbowkit'
2import { configureChains, createConfig } from 'wagmi'
3import { holesky, sepolia } from 'wagmi/chains'

Import the blockchains the application supports. You can see the list of supported chains in the viem github(opens in a new tab).

1import { publicProvider } from 'wagmi/providers/public'
3const walletConnectProjectId = 'c96e690bb92b6311e8e9b2a6a22df575'

To be able to use WalletConnect(opens in a new tab) you need a project ID for your application. You can get it on in a new tab).

1const { chains, publicClient, webSocketPublicClient } = configureChains(
2 [ holesky, sepolia ],
3 [
4 publicProvider(),
5 ],
8const { connectors } = getDefaultWallets({
9 appName: 'My wagmi + RainbowKit App',
10 chains,
11 projectId: walletConnectProjectId,
14export const config = createConfig({
15 autoConnect: true,
16 connectors,
17 publicClient,
18 webSocketPublicClient,
21export { chains }
Mostra tutto

Adding another blockchain

These days there are a lot of L2 scaling solution(opens in a new tab), and you might want to support some that viem does not support yet. To do it, you modify src/wagmi.ts. These instructions explain how to add Redstone Holesky(opens in a new tab).

  1. Import the defineChain type from viem.

    1import { defineChain } from 'viem'
  2. Add the network definition.

    1const redstoneHolesky = defineChain({
    2 id: 17_001,
    3 name: 'Redstone Holesky',
    4 network: 'redstone-holesky',
    5 nativeCurrency: {
    6 decimals: 18,
    7 name: 'Ether',
    8 symbol: 'ETH',
    9 },
    10 rpcUrls: {
    11 default: {
    12 http: [''],
    13 webSocket: ['wss://'],
    14 },
    15 public: {
    16 http: [''],
    17 webSocket: ['wss://'],
    18 },
    19 },
    20 blockExplorers: {
    21 default: { name: 'Explorer', url: '' },
    22 },
    Mostra tutto
  3. Add the new chain to the configureChains call.

    1 const { chains, publicClient, webSocketPublicClient } = configureChains(
    2 [ holesky, sepolia, redstoneHolesky ],
    3 [ publicProvider(), ],
    4 )
  4. Ensure that the application knows the address for your contracts on the new network. In this case, we modify src/components/Greeter.tsx:

    1const contractAddrs : AddressPerBlockchainType = {
    2 // Holesky
    3 17000: '0x432d810484AdD7454ddb3b5311f0Ac2E95CeceA8',
    5 // Redstone Holesky
    6 17001: '0x4919517f82a1B89a32392E1BF72ec827ba9986D3',
    8 // Sepolia
    9 11155111: '0x7143d5c190F048C8d19fe325b748b081903E3BF0'
    Mostra tutto


Of course, you don't really care about providing a user interface for Greeter. You want to create a user interface for your own contracts. To create your own application, run these steps:

  1. Specify to create a wagmi application.

    1pnpm create wagmi
  2. Name the application.

  3. Select React framework.

  4. Select the Vite variant.

  5. You can add Rainbow kit(opens in a new tab).

Now go and make your contracts usable for the wide world.

Ultima modifica: @wackerow(opens in a new tab), 13 marzo 2024

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