<h2>FAQs</h2>
<h3><strong>How do I get started with Ethereum / Smart Contract development?</strong></h3>
<p>This really depends on your prior experience with Blockchain technologies such as Smart Contracts and programming languages.</p>
<p>If you have little experience with Blockchains and their inner working we'd advise to start by reading through some of the "<a href="https://wiki.cryptodevhub.io/general-resources">General Resources</a>" such as Binance Academy or CoinMarketCap Academy.</p>
<p>If you're already familiar with the Blockchain but lack Smart Contract / dApp development experience you'll find a full curriculum in our "<a href="https://wiki.cryptodevhub.io/ethereum-evm/getting-started">Getting Started</a>" section.</p>
<p>Seasoned dApp developers and folks familiar with other Blockchain ecosystems can start by looking through our "<a href="https://wiki.cryptodevhub.io/ethereum-evm/tutorials">Tutorials</a>" list.</p>
<h3>How do I run a local Ethereum Blockchain Node?</h3>
<p>Generally speaking there are 2 popular solutions for running local Ethereum Blockchain Nodes.</p>
<h4>Ganache CLI</h4>
<p>The Ganache project is part of the Truffle Suite, a popular choice for Smart Contract- and dApp development on the Ethereum Blockchain. While Truffle provides a fully-fledged development environment which makes it simple to compile, test and deploy Smart Contracts and dApp, Ganache can be thought of an extension to Truffle which makes it possible to run your code locally.</p>
<p>Ganache comes in 2 flavors: Ganache (a desktop app) and Ganache CLI. We'd recommend to use the CLI version as it integrates nicely with other tools such as Truffle or Hardhat and can be automated and therefore leveraged in e.g. CI / CD systems or end-to-end tests.</p>
<p>Installing <a href="https://github.com/trufflesuite/ganache-cli">Ganache CLI</a> is as simple as running <code>npm install --save-dev ganache-cli</code>. This will add Ganache as a development dependency to your current project.</p>
<p>Once installed you can start a local Ethereum Node via <code>npx ganache-cli</code>.</p>
<p>Notice that Ganache CLI has a slew of <a href="https://github.com/trufflesuite/ganache-cli#options">options</a> you can pass into the command. What we'd recommend is to e.g. use the <code>-d</code> / <code>--deterministic</code> flag such that deterministic accounts are generated: <code>npx ganache-cli --deterministic</code>.</p>
<h4>Hardhat Network</h4>
<p>If you're using Hardhat as your Ethereum dApp development framework you're already in possession of an Ethereum Node you can run locally.</p>
<p>The Hardhat development environment ships with the <a href="https://hardhat.org/hardhat-network/">Hardhat Network</a>, a local Ethereum network specifically designed for development. In fact, every time you ran a Hardhat test you leveraged such network behind the scenes.</p>
<p>Running a local Ethereum Blockchain Node is as simple as running <code>npx hardhat node</code>.</p>
<p>There's also a lot you can configure when using Hardhat Networks. We'd advise to read through the <a href="https://hardhat.org/hardhat-network/">thorough documentation</a> to learn more about its capabilities.</p>
<h3>How do I get realtime data from the Ethereum Blockchain?</h3>
<p>Almost all Ethereum JSON-RPC API implementations support the <code>eth_subscribe</code> method which makes it possible to create a so called subscription to listen to events emitted on the Blockchain. You can use this method in combination with WebSockets to get notified when something happens on the blockchain.</p>
<p>The following 2 code examples demonstrate how you can listen for new blocks being created. Browsing through your library's documentation you should also find examples on how to listen for e.g. contract events.</p>
<h4>Web3.js</h4>
<pre><code>const Web3 = require('web3')

const URL = 'wss://example.com'
const provider = new Web3.providers.WebsocketProvider(URL)

const web3 = new Web3(provider)

web3.eth.subscribe('newBlockHeaders').on('data', (block) => {
  // A new block was created
  console.log(block)
})
</code></pre>
<h4>Ethers</h4>
<pre><code>const { ethers } = require('ethers')

const provider = new ethers.providers.JsonRpcProvider()

provider.on('block', (blockNumber) => {
  // A new block was created
  console.log(blockNumber)
})
</code></pre>
<p><strong>Note</strong>: While you could achieve the same functionality by continously requesting new data from the Ethereum blockchain every X seconds (called "pull"), using subscriptions (called "push") is way more compute- and bandwidth efficient.</p>
<h3>How do I call a deployed Smart Contract?</h3>
<p>To call an already deployed Smart Contract you need 2 things:</p>
<ol>
<li>The Smart Contract address</li>
<li>The Smart Contract ABI</li>
</ol>
<p>Both information can usually be found in the projects documentation or via block explorers such as Etherscan.</p>
<p><a href="https://etherscan.io/address/0x7a250d5630b4cf539739df2c5dacb4c659f2488d">Here</a> fore example is the "Uniswap Router V2" Smart Contract on Etherscan. If you click on "Contract" and scroll down you'll find a section "Contract ABI" which shows you the Smart Contract's ABI. The address can be found at the top of the page or in the URL in your browser's address bar.</p>
<p>Having both information you can now call the Smart Contract via your client library of choice. The following are examples in both Web3.js and Ethers.</p>
<h4>Web3.js</h4>
<pre><code>const Web3 = require("web3");

const URL = "wss://example.com"
const provider = new Web3.providers.WebsocketProvider(URL)

const web3 = new Web3(provider);

const address = "0x1234567890"; // &#x3C;-- The Smart Contract address
const abi = [{ ... }]; // &#x3C;-- The Smart Contract ABI

// The arguments for your function call
const arg1 = "Foo";
const arg2 = "Bar";

const instance = new web3.eth.Contract(abi, address);

const result = await instance.methods.myFunc(arg1, arg2).call();

console.log(result);
</code></pre>
<h4>Ethers</h4>
<pre><code>const { ethers } = require("ethers");

const provider = new ethers.providers.JsonRpcProvider();

const address = "0x1234567890"; // &#x3C;-- The Smart Contract address
const abi = [{ ... }]; // &#x3C;-- The Smart Contract ABI

// The arguments for your function call
const arg1 = "Foo";
const arg2 = "Bar";

const instance = new ethers.Contract(address, abi, provider);

const result = await instance.myFunc(arg1, arg2);

console.log(result);
</code></pre>
<h3>Where do I get the testnet address of token X?</h3>
<p>The best resource to find the corresponding token addresses is in the projects documentation. Let's take a look at an example.</p>
<p>If you're working on an integration with Kyber which is deployed on the Ropsten testnet and uses DAI behind the scenes you'd find the corresponding address <a href="https://developer.kyber.network/docs/RopstenEnvGuide/#token-addresses">here</a>. In this case the DAI address is <code>0xaD6D458402F60fD3Bd25163575031ACDce07538D</code>.</p>
<p>Now let's say that you're extending your dApp's capabilities via an integration with Aave v1. Looking through their documentation you'll find the Ropsten testnet addresses <a href="https://docs.aave.com/developers/v/1.0/deployed-contracts/deployed-contract-instances">here</a>. DAI has the address <code>0xf80A32A835F79D7787E8a8ee5721D0fEaFd78108</code>.</p>
<p>Now if you pay close attention you'll notice that bot addresses are different and this is important to remember when working with testnets. Given that DAI and other tokens have no value on testnets they're usually deployed multiple times. More often than not you'll encounter project's using different deployment, making it nearly impossible for your to test complex interactions based on tokenized assets.</p>
<p>If you're running into this problem you should skip testnets and switch to a mainnet forking + account impersonating strategy, both of which are discussed in the following two questions.</p>
<h3>Is there a way to simulate real transactions on mainnet?</h3>
<p>If you're using a local Ethereum Blockchain Node for development like Ganache CLI or Hardhat Network you can add an option which enables "mainnet forking".</p>
<p>Forking mainnet means that your local Blockchain Node will take a snapshot of the Ethereum mainnet including account and Smart Contract state and save it offline. Any subsequent interaction with your local Blockchain Node will be based off of this snapshot without affecting the real Ethereum mainnet.</p>
<p>Using "mainnet forking" you can then interact with all the protocols already deployed on Ethereum mainnet via their respective "real" addresses and Smart Contract ABIs. But instead doing "real" transactions you run them locally with fake ETH. This capability is super powerful as it makes it super simple and efficient to test real-world use cases.</p>
<p>The following examples show you how you can do "mainnet forking" with Ganache CLI and Hardhat Network. For more information we'd encourage you to browse through their official docs about "mainnet forking" (<a href="https://github.com/trufflesuite/ganache-cli#readme">Ganache CLI</a>, <a href="https://hardhat.org/guides/mainnet-forking.html">Hardhat Network</a>).</p>
<p><strong>Note</strong>: You can also add another option to start the forking process at block X. This is very useful if you're relying on mainnet forking in your tests as it makes testing via the forking mechanism deterministic.</p>
<h4>Ganache CLI</h4>
<pre><code>npx ganache-cli --fork https://cloudflare-eth.com
</code></pre>
<h4>Hardhat Network</h4>
<pre><code>npx hardhat node --fork https://cloudflare-eth.com
</code></pre>
<h3>How do I mint token X for testing?</h3>
<p>The easiest way to mint a token for testing is to not mint it at all but impersonate someone else's account. While this sounds ethical questionable it's a very powerful strategy with zero harm involved for the impersonated account holder.</p>
<p>What you'd do in this case is you'd run a local Ethereum Blockchain Node via "mainnet forking" (see above) while also passing in a parameter to unlock an account of your choice. This account should hold some amount of the token you're interested in during your testing.</p>
<p>You can then spend any token of the unlocked account in your local Blockchain Node instance without possessing its private key.</p>
<p>The following are examples which show how you'd start a Ganache CLI as well as Hardhat Network instance with account impersonation enabled. Please consult the corresponding tooling documentation for more information (<a href="https://github.com/trufflesuite/ganache-cli#readme">Ganache CLI</a>, <a href="https://hardhat.org/guides/mainnet-forking.html">Hardhat Network</a>).</p>
<h4>Ganache CLI</h4>
<pre><code>npx ganache-cli --fork https://cloudflare-eth.com --unlock 0x1234567890
</code></pre>
<h4>Hardhat Network</h4>
<pre><code>npx hardhat node --fork https://cloudflare-eth.com
</code></pre>
<pre><code>await hre.network.provider.request({
  method: 'hardhat_impersonateAccount',
  params: ['0x1234567890']
})
</code></pre>
<h3>How much does a Smart Contract audit cost?</h3>
<p>As it's usually the case, this really depends on the size and complexity of your project. Most auditing firms charge per time spent inspecting the contract(s).</p>
<p>Larger projects (more lines-of-code) usually take longer to audit. The same applies to complex projects. An ERC-20 contract for example is easier to audit compared to a new, non-standard custom token implementation.</p>
<p>Here are some rough estimates taken from a <a href="https://medium.com/iearn/things-i-wish-i-knew-before-building-ethereum-defi-dapps-cd6bf0f07a16">blog post</a> written by Andre Cronje. The code in question is 359 LOC written in Solidity:</p>
<ul>
<li>trailofbits.com $16.000 1 week, 1 person</li>
<li>cryptomaniacs $10.000 2 weeks, 2 people</li>
<li>sigmaprime.io $27.500 11 days, 2 people</li>
</ul>
<p>Andre concludes that bigger projects usually cost in the range of $50.000 to audit.</p>
<p><strong>Note</strong>: There's projects such as <a href="https://immunefi.com/">Immunefi</a> which will help you set up and coordinate a bug bounty program for your Smart Contract project. It's not the same as an audit but lets you work together with white hat hackers to uncover security vulnerabilities.</p>
<h3>I'm getting an "Internal JSON RPC error" when working with Metamask</h3>
<p>Are you running an Ethereum Node via Ganache CLI or Hardhat Network for local development? If true, you're very likely reusing an old network configuration via Metamask.</p>
<p>Metamask keeps track of nonces which are auto-incrementing numbers assigned to every transaction. And while your new Node starts with nonces at 0, Metamask still stores the nonce state of the old Node configuration.</p>
<p>To solve this problem all you have to do is <a href="https://metamask.zendesk.com/hc/en-us/articles/360015488891-How-to-reset-your-wallet">reset your Metamask account</a>. This has to be done every time you reuse an existing Metamask network configuration with a new Ethereum Node.</p>


Frequently asked questions about Smart Contract development on the Ethereum Virtual Machine (EVM).

Ethereum Virtual Machine (EVM) - Frequently Asked Questions

<h2>Welcome</h2>
<p>Welcome to the <strong>CryptoDevHub</strong> project, the central place where we collect and catalog the best content for Blockchain development and Crypto technologies.</p>
<p>At the top you can find the main navigation which links to various pages with useful resources for different topics.</p>
<p>If you go back to the <a href="/">homepage</a> you'll find curated collections with resources useful for your Blockchain- and Smart Contract development journey.</p>
<h3>Blockchain Development Tutorial</h3>
<p>If you're looking for a "Getting Started" guide to dive straight into the wonderful world of Blockchain development, you might want to check out our <a href="/blockchain-development-tutorial">"Blockchain Development Tutorial"</a> which teaches you everything you need to know to build your first Web3 dApp.</p>
<h3>Let's stay in touch</h3>
<p>Also make sure to <a href="https://twitter.com/cryptodevhub">follow us on Twitter</a> and join our <a href="https://cryptodevhub.io/discord">Discord server</a> to connect with other developers and get frequent updates about the Blockchain- and Crypto Technology space.</p>
<h3>CryptoDevHub Community</h3>
<p>This website is just one part of our <a href="https://cryptodevhub.io">CryptoDevHub</a> community. If you haven't already you might also want to join our <a href="https://cryptodevhub.io/discord">Discord server</a>.</p>


Start Here

<h2>Getting Started</h2>
<h3>0. Learn JavaScript Basics (Optional)</h3>
<p>A typical dApp is a combination of a Frontend (what the user sees and interacts with) and a Backend (usually your Smart Contracts deployed on the Ethereum Blockchain).</p>
<p>Given that Frontends as well as the vast majority of Ethereum's Smart Contract development tooling is written in JavaScript it's a worthwhile endeavour to brush up your JavaScript skills before diving deeper into Solidity and dApp development. If you're new to JavaScript or need a quick refresher you'll find a good resource in the <a href="https://www.codecademy.com/learn/introduction-to-javascript">Learn JavaScript</a> course by Codecademy.</p>
<p><strong>Protip</strong>: Don't spend too much time on intermediate and advanced topics. Knowing the basics of JavaScript should be more than enough to get you going.</p>
<h3>1. Work through CryptoZombies</h3>
<p>The best way to learn something new is through a structured curriculum which walks you through the big ideas and major concepts in a guided fashion. Ideally as frictionless as possible.</p>
<p><a href="https://cryptozombies.io/">CryptoZombies</a> is one of the best resources for every aspiring Ethereum dApp / Solidity developer. In it you'll build your own NFT-style Zombie game and learn all the important concepts of EVM development and the Solidity programming language. The best part is that it's completely browser based, so you don't need to install and configure anything.</p>
<p>While you can work through the whole course we'd advice to focus on the first 2 sections about Solidity Smart Contract development.</p>
<p>If you're interested in writing Smart Contracts with Vyper you should check out <a href="https://vyper.fun/">Vyper.fun</a> which is another great interactive resource focused on the Vyper programming language.</p>
<h3>2. Learn about Truffle Suite / Hardhat</h3>
<p>While you can install and setup all the different toolings such as a Solidity / Vyper compiler from scratch doesn't mean you should.</p>
<p>Luckily enough others jumped in and built whole developer tool suites such as the <a href="https://www.trufflesuite.com/">Truffle Suite</a> or <a href="https://hardhat.org/">Hardhat</a>. Take some time to browse through both projects and play around with their solutions.</p>
<p>Once you're familiar with both, pick your favorite development stack and stick with it (at least for the time being). For beginners Truffle is usually the best choice as it comes with "batteries included". Intermediate / Advanced users will find more flexibility in Hardhat.</p>
<h3>3. Build your first dApp</h3>
<p>Given that you're now familiar with some basic Solidity / Vyper and the developer toolings in the Ethereum dApp space it's time to work on your first "real" project.</p>
<p>If you're already familiar with Software- and JavaScript development in general you might want to jump straight into your own first project without any further guidance.</p>
<p>For others, it might be worthwhile to follow an online tutorial like <a href="https://www.dappuniversity.com/articles/the-ultimate-ethereum-dapp-tutorial">this one</a> from "Dapp University".</p>
<p>It isn't super important what you're building as long as you focus your efforts on writing code, rather than consuming more blog posts and YouTube videos.</p>
<h3>4. Build more dApps</h3>
<p>Once done with your very first dApp, you surely ran into various roadblocks you didn't anticipate beforehand. While having guardrails in the beginning is a necessity, it's being on your own where true growth happens and the learnings materialize.</p>
<p>Given that, you should spend some more time building dApps end-to-end. This time around <strong>without any guardrails</strong>. Start with a clean slate and work your way through the whole process. Start with the Smart Contract logic, connect it to a frontend and ensure that everything works by writing unit- and integration tests.</p>
<p>It's absolutely ok to check references and existing code to get you unstuck but focus on trying to solve the problem on your own before consulting other resources.</p>
<p>"What should I build?". That's a common and valid question given that we should be focusing on deliberate practice rather than rote repetition. The following is a list of dApp ideas you can draw from (ordered by difficulty: Easy - Hard):</p>
<h4>Voting (Easy)</h4>
<p>A voting dApp where users can vote for candidates. Every user should only be able to vote once.</p>
<h4>CRUD (Easy)</h4>
<p>A CRUD (Create, Read, Update, Delete) dApp where users can create posts, read posts, update existing posts and remove posts. You should track the ownership of posts such that only owners can update and remove posts.</p>
<h4>Escrow (Medium)</h4>
<p>Write an escrow Smart Contract which helps users facilitate transactions. User A should be able to deposit funds in the Smart Contract while user B should be able to withdraw the previously deposited funds from user A.</p>
<p>You can add a timelock-like feature where funds will be automatically sent back to the depositor after a certain number of blocks has been mined and the funds haven't been withdrawn by user B.</p>
<h4>Lottery (Medium)</h4>
<p>Create a Lottery dApp where users can buy tickets to participate. The money they spent on the tickets is added to a pool. After a pre-defined number of blocks has been mined, a winner is chosen randomly who receives all the funds from the pool.</p>
<p><strong>Note</strong>: You can use a simple random number generation schemes derived from block hashes. There's no need to integrate a VRF via an Oracle (although you might want to try that as well).</p>
<h4>Multi-Sig Wallet (Hard)</h4>
<p>Implement a Multi-Sig Wallet where multiple signatures are necessary to create a transaction and spend coins.</p>
<h4>Payment Channel (Hard)</h4>
<p>Implement a payment channel which coordinates transactions between user A and user B off-chain and settles them on-chain. To keep it simple you can start with a Uni-directional Payment Channel and extend it later on to be Bi-directional.</p>
<h4>Flash Loans (Hard)</h4>
<p>Implement a Flash Loan strategy in which you borrow funds, exploit an arbitrage opportunity and repay the borrowed funds while pocketing the difference, all within one transactions.</p>
<p>To help you get started with this one you might want to read how Flash Loans work by reading through the <a href="https://money-legos.studydefi.com/">Money Legos</a> project documentation for the <a href="https://money-legos.studydefi.com/#/aave?id=flashloans">Aave</a> and <a href="https://money-legos.studydefi.com/#/dydx?id=flashloans-on-dydx">dYdX</a> DeFi protocols.</p>
<p>Also learn about "Mainnet Forking" capabilities of Ganache CLI and Hardhat Network. That way you don't need to continually re-deploy your Smart Contract on a testnet.</p>
<p><strong>Protip</strong>: It's more than enough to build 2-3 dApps end-to-end. Move on to the next step the minute "everything clicks".</p>
<h3>5. Learn to use OpenZeppelin's Smart Contracts</h3>
<p>Writing secure Solidity / Vyper code can be a tough endeavour. Luckily the folks at OpenZeppelin created and maintain a <a href="https://github.com/OpenZeppelin/openzeppelin-contracts">GitHub repository</a> with audited Smart Contract building blocks you can incorporate in your projects.</p>
<p>The full documentation for all the different Smart Contracts can be found <a href="https://docs.openzeppelin.com/contracts">here</a>.</p>
<h3>6. Get to know ERC standards</h3>
<p><a href="https://eips.ethereum.org/erc">ERC's</a> are Ethereum Improvement Proposals in which different ideas to improve the Ethereum Blockchain are discussed. One of the most famous ERC's are ERC-20 and ERC-721, both of which describe Token standards.</p>
<p>Read through the ERC-20 and ERC-721 standards and build a dApp which leverages those.</p>
<p><strong>Protip</strong>: OpenZeppelin's Smart Contracts package should help you in doing so.</p>
<h3>7. Learn about Smart Contract security</h3>
<p>We already touched on the importance of Smart Contract security when it comes to dApp and Blockchain development. Take some time to read through our <a href="https://wiki.cryptodevhub.io/ethereum-evm/security">Security</a> page and learn more about recent hacks and potential pitfalls.</p>
<h3>8. Join Communities</h3>
<p>Learning never stops and is best nurtured via thoughtful discussions and working with others. Our <a href="https://wiki.cryptodevhub.io/ethereum-evm/communities">Communities</a> page will show you all the Ethereum communities you might want to join.</p>
<p>You should also consider contributing back to your favorite Open Source projects (or start your own). Not only do you provide immediate value, working on Open Source is one of the best ways to solidify your knowledge and grow as an engineer.</p>
<h3>Bonus: Streamline your DX</h3>
<p>While Truffle and Hardhat help you getting up to speed there's more tooling you can leverage to streamline your overall workflow and Developer Experience.</p>
<p>Yos Riady's article "<a href="https://yos.io/2019/11/10/smart-contract-development-best-practices/">Best Practices for Smart Contract Development</a>" is a treasure trove of great advice on how to setup a professional development environment.</p>
<p>In addition to that you should also check out Paul Berg's <a href="https://github.com/PaulRBerg/create-eth-app">create-eth-app</a> or Austin Griffith's <a href="https://github.com/austintgriffith/scaffold-eth">scaffold-eth</a> projects which will help you getting started even faster.</p>
<p>You can find a more thorough list of useful tools on our <a href="https://wiki.cryptodevhub.io/ethereum-evm/tools">Tools</a> page.</p>


Learn Solidity and Smart Contract development to build dApps on Ethereum-Virtual-Machine (EVM) based chains.

Article

Ethereum Virtual Machine (EVM) - Frequently Asked Questions

Start Here

Blockchain Development Tutorial