Hardhat comes built-in with Hardhat Network, a local Ethereum network node designed for development. It allows you to deploy your contracts, run your tests and debug your code, all within the confines of your local machine.
It runs as either an in-process or stand-alone daemon, servicing JSON-RPC and WebSocket requests.
By default, it mines a block with each transaction that it receives, in order and with no delay.
It's backed by the
@ethereumjs/vm EVM implementation, the same one used by ganache, Remix and Ethereum Studio.
By default, if you're using Hardhat, then you're already using Hardhat Network.
When Hardhat executes your tests, scripts or tasks, an in-process Hardhat Network node is started automatically, and all of Hardhat's plugins (ethers.js, web3.js, Waffle, Truffle, etc) will connect directly to this node's provider.
There's no need to make any changes to your tests or scripts.
Hardhat Network is simply another network. If you wanted to be explicit, you could run, for example,
npx hardhat run --network hardhat scripts/my-script.js.
Alternatively, Hardhat Network can run in a stand-alone fashion so that external clients can connect to it. This could be MetaMask, your Dapp front-end, or a script. To run Hardhat Network in this way, run:
$ npx hardhat node Started HTTP and WebSocket JSON-RPC server at http://127.0.0.1:8545/ Accounts ======== Account #0: 0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266 (10000 ETH) Private Key: 0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80 Account #1: 0x70997970c51812dc3a010c7d01b50e0d17dc79c8 (10000 ETH) Private Key: 0x59c6995e998f97a5a0044966f0945389dc9e86dae88c7a8412f4603b6b78690d ...
This will start Hardhat Network, and expose it as a JSON-RPC and WebSocket server.
Then, just connect your wallet or application to
If you want to connect Hardhat to this node, you just need to run using
Do not send mainnet Ether to the account addresses shown by
hardhat node. Those addresses and private keys are deterministic: they are the same for all Hardhat users. Accordingly, those private keys are well known, so there are probably bots monitoring these addresses on mainnet, waiting to withdraw any funds sent to them. If you add any of those accounts to a wallet (eg Metamask), be very careful to avoid sending any mainnet Ether to them: consider naming the account something like "Hardhat - Unsafe" in order to prevent any mistakes.
Hardhat Network has first-class Solidity support. It always knows which smart contracts are being run, what they do exactly and why they fail.
This is an example of a Hardhat Network exception using
Error: Transaction reverted: function selector was not recognized and there's no fallback function at ERC721Mock.<unrecognized-selector> (contracts/mocks/ERC721Mock.sol:9) at ERC721Mock._checkOnERC721Received (contracts/token/ERC721/ERC721.sol:334) at ERC721Mock._safeTransferFrom (contracts/token/ERC721/ERC721.sol:196) at ERC721Mock.safeTransferFrom (contracts/token/ERC721/ERC721.sol:179) at ERC721Mock.safeTransferFrom (contracts/token/ERC721/ERC721.sol:162) at TruffleContract.safeTransferFrom (node_modules/@nomiclabs/truffle-contract/lib/execute.js:157:24) at Context.<anonymous> (test/token/ERC721/ERC721.behavior.js:321:26)
Hardhat Network always knows why your transaction or call failed, and it uses this information to make debugging your contracts easier.
When a transaction fails without a reason, Hardhat Network will create a clear error message in the following cases:
Calling a non-payable function with ETH
Sending ETH to a contract without a payable fallback or receive function
Calling a non-existent function when there's no fallback function
Calling a function with incorrect parameters
Calling an external function that doesn't return the right amount of data
Calling an external function on a non-contract account
Failing to execute an external call because of its parameters (e.g. trying to send too much ETH)
Calling a library without
Incorrectly calling a precompiled contract
Trying to deploy a contract that exceeds the bytecode size limit imposed by EIP-170
Hardhat Network allows you to print logging messages and contract variables by calling
console.log() from your Solidity code.
To use it, you simply import
hardhat/console.sol and call it. It implements the same formatting options that can be found in Node.js'
console.log, which in turn uses
util.format. For example:
console.log("Changing owner from %s to %s", currentOwner, newOwner).
It always works, regardless of the call or transaction failing or being successful. And, because it's implemented in standard Solidity, it works with any tool or library, emitting log entries where it's fully supported — Hardhat Network, Remix, and Tenderly — and falling back gracefully to a no-op everywhere else — Remix, Waffle, Truffle, etc — though it does consume a small amount of gas on live networks.
Hardhat Network has the ability to copy the state of the mainnet blockchain into your local environment, including all balances and deployed contracts. This is known as "forking mainnet."
In a local environment forked from mainnet, you can execute transactions to invoke mainnet-deployed contracts, or interact with the network in any other way that you would with mainnet. In addition, you can do anything supported by a non-forked Hardhat Network: see console logs, get stack traces, or use the default accounts to deploy new contracts.
More generally, Hardhat Network can be used to fork any network, not just mainnet. Even further, Hardhat Network can be used to fork any EVM-compatible blockchain, not just Ethereum.
There are other things you can do with a forked Hardhat Network. Check our guide to learn more.
Hardhat Network can be configured to automine blocks, immediately upon receiving each transaction, or it can be configured for interval mining, where a new block is mined periodically, incorporating as many pending transactions as possible.
You can use one of these modes, both or neither. By default, only the automine mode is enabled.
If neither mining mode is enabled, no new blocks will be mined, but you can manually mine new blocks using the
evm_mine RPC method. This will generate a new block that will include as many pending transactions as possible.
For more details on mining modes and mempool behavior, see Mining Modes.
Hardhat Network uses its tracing infrastructure to offer rich logging that will help you develop and debug smart contracts.
For example, a successful transaction and a failed call would look like this:
eth_sendTransaction Contract deployment: Greeter Contract address: 0x8858eeb3dfffa017d4bce9801d340d36cf895ccf Transaction: 0x7ea2754e53f09508d42bd3074046f90595bedd61fcdf75a4764453454733add0 From: 0xc783df8a850f42e7f7e57013759c285caa701eb6 Value: 0 ETH Gas used: 568851 of 2844255 Block: #2 - Hash: 0x4847b316b12170c576999183da927c2f2056aa7d8f49f6e87430e6654a56dab0 console.log: Deploying a Greeter with greeting: Hello, world! eth_call Contract call: Greeter#greet From: 0xc783df8a850f42e7f7e57013759c285caa701eb6 Error: VM Exception while processing transaction: revert Not feeling like it at Greeter.greet (contracts/Greeter.sol:14) at process._tickCallback (internal/process/next_tick.js:68:7)
This logging is enabled by default when using Hardhat Network's node (i.e.
npx hardhat node), but disabled when using the in-process Hardhat Network provider. See Hardhat Network's config to learn more about how to control its logging.
You can get debug traces of already-mined transactions using the
debug_traceTransaction RPC method. The returned object has a detailed description of the transaction execution, including a list of steps describing each executed opcode and the state of the EVM at that point.
If you are using mainnet forking with an archive node, you can get traces of transactions from the remote network even if the node you are using doesn't support
For more details, see the reference documentation for this method.
This has been just a high-level explanation of what Hardhat Network is. To dig deeper, see the Reference documentation.