Smock: The powerful mocking tool for Hardhat
Features of smock and how to use them with examples
We’ve covered mocking contracts before, but now there’s an additional great tool available: smock. It simplifies the mocking process greatly and also gives you more testing power. You’ll be able to change the return values for functions as well as changing internal contract storage directly!
How cool is that?
How to set up ?
Requirements: Currently only Hardhat with Waffle is supported. You can follow the setup here. If you use Truffle, you can take a look at mock-contract or wait for future support (see bottom).
The smock tool is available as plugin. You can install it via Npm using:
$ npm install @eth-optimism/smock --save-devNow you just have to add the storage plugin inside hardhat.config.js:
require('@eth-optimism/smock/build/src/plugins/hardhat-storagelayout')That's all, let's go ahead and use it.
smockit
The first functionality of changing the return values is available via smockit. With this tool you can mock function calls:
- - asserting call count and call data
- - returning mocked data
- - reverting with data
Will the code in the function be executed?
- No, there will be no code of the mocked function executed.
Can I mock an internal function call?
- No, that's currently not supported. Only external function calls can be mocked. But this might change in the future (see bottom).
smockit is available via import:
const { smockit } = require("@eth-optimism/smock");smoddit
The second functionality of changing internal storage is available via smoddit. With this tool you can directly change the contract storage:
- modify primitive types directly
- change data in mappings
- modify stored structs
smoddit is available via import:
const { smoddit } = require("@eth-optimism/smock");Let's see an example
MyERC20.sol
Imagine we have an ERC-20 contract with a mintUpTo function. This function will mint whatever amount is required to mint to get the passed address to the requested balance amount.
This function as you can see is onlyOwner. Now imagine a complex system where the onlyOwner is really just another contract who's allowed to call this function. Let's see an example second contract for this...
pragma solidity ^0.7.0;
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
contract MyERC20 is ERC20, Ownable {
constructor() ERC20("MyERC20", "MYE") {}
function mintUpTo(
address to, uint256 amount
) external onlyOwner returns (uint256) {
uint256 currentBalance = balanceOf(to);
if (currentBalance >= amount) return 0;
uint256 mintBalance = amount - currentBalance;
_mint(to, mintBalance);
return mintBalance;
}
}pragma solidity ^0.7.0;
import "hardhat/console.sol";
import "./MyERC20.sol";
contract MyOtherContract {
MyERC20 private myERC20;
constructor(MyERC20 myERC20_) {
myERC20 = myERC20_;
}
function myOtherFunction(
address to, uint256 amount
) external returns (bool) {
// do stuff
uint256 mintAmount = myERC20.mintUpTo(to, amount);
console.log("The minted amount was", mintAmount);
// do more stuff
}
}MyOtherContract.sol
In this contract we call the mintUpTo function. Remember this will only work if the contract is actually the owner. This is the intended behavior we want in a mainnet setup. But wouldn't it be useful if we could avoid this just for our unit-tests?
Figuring out the system state in where MyOtherContract is actually the owner of myERC20 might require some complex initialization steps. Let's avoid this by mocking the mintUpTo function...
Mocking mintUpTo via smockit
We can first create the smocked contract using:
smockit(myERC20)This will create a copy of the contract with its own address. The address is actually just an empty account without any code. So don't call any actual functions here, only mocked ones.
So to mock our function we use:
MyMockContract.smocked.mintUpTo
.will.return.with(mockedMintAmount)We now pass the mocked contract address to our MyOtherContract. And as you can see, we can call myOtherFunction even though it's not the owner of myERC20.
On top we can actually check and verify the calls to mintUpTo. So in our unit test we can assert that the mintUpTo does indeed happen with the expected call parameters available via:
MyMockContract.smocked.mintUpTo.callsconst { expect } = require("chai");
const { smoddit, smockit } = require("@eth-optimism/smock");
const { BigNumber } = require("ethers");
describe("My ERC20 and other contract", function () {
let myERC20;
beforeEach(async () => {
const MyERC20 = await ethers.getContractFactory("MyERC20");
myERC20 = await MyERC20.deploy();
await myERC20.deployed();
});
it("call mint up in my ERC20", async function () {
const MyMockContract = await smockit(myERC20);
const MyOtherContract = await ethers.getContractFactory(
"MyOtherContract"
);
const myOtherContract = await MyOtherContract.deploy(
MyMockContract.address
);
const mockedMintAmount = 30;
MyMockContract.smocked.mintUpTo
.will.return.with(mockedMintAmount);
const to = "0xdAC17F958D2ee523a2206206994597C13D831ec7";
const amount = 100;
await myOtherContract.myOtherFunction(to, amount);
expect(MyMockContract.smocked.mintUpTo.calls.length)
.to.be.equal(1);
expect(MyMockContract.smocked.mintUpTo.calls[0].to)
.to.be.equals(to);
expect(
MyMockContract.smocked.mintUpTo.calls[0].amount.toString()
).to.equal(amount.toString());
});
});it("call mint up in my ERC20", async function () {
const MyModifiableERC20Factory = await smoddit("MyERC20");
const MyModifiableERC20 = await MyModifiableERC20Factory.deploy();
const MyOtherContract = await ethers.getContractFactory(
"MyOtherContract"
);
const myOtherContract = await MyOtherContract.deploy(
MyModifiableERC20.address
);
await MyModifiableERC20.transferOwnership(myOtherContract.address);
const to = "0xdAC17F958D2ee523a2206206994597C13D831ec7";
const transferAmount = 100;
const mockedBalance = BigNumber.from("10");
MyModifiableERC20.smodify.put({
_balances: {
[to]: mockedBalance,
},
});
expect((await MyModifiableERC20.balanceOf(to)).toString()).to.equal(
mockedBalance.toString()
);
await myOtherContract.myOtherFunction(to, transferAmount);
expect((await MyModifiableERC20.balanceOf(to)).toString()).to.equal(
transferAmount.toString()
);
});Changing token balance via smoddit
We can also modify the internal storage of a contract via smoddit. Create a contract factory and use it to deploy the contract:
const Factory = await smoddit("MyERC20");
const Contract = await Factory.deploy()The resulting contract can be modified directly. In our case we can for example change the ERC-20 balance of the contract:
Contract.smodify.put({
_balances: {
[to]: mockedBalance,
},
});An Optimistic Smock Future
I've reached out to Kelvin Fichter, the author of Smock and part of the Optimism team. As of right now you need Waffle and Buidler to use the tool. But in the next version it's planned to add support for Truffle.
And what I'm particularly excited about, we might get support for mocking internal functions. Instead of creating an empty account for the mocked address, the contract would have actual code behind the address. This would greatly improve the functionality. Stay tuned for further updates!
Solidity Developer
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Why Buidler? Proper debugging is a pain with Truffle. Events are way too difficult to use as logging and they don't even work for reverted transactions (when you would need them most). Buidler gives you a console.log for your contracts which is a game changer. And you'll also get stack traces...
Contract factories and clones
How to deploy contracts within contracts as easily and gas-efficient as possible
The factory design pattern is a pretty common pattern used in programming. The idea is simple, instead of creating objects directly, you have an object (the factory) that creates objects for you. In the case of Solidity, an object is a smart contract and so a factory will deploy new contracts for...
How to use IPFS in your Dapp?
Using the interplanetary file system in your frontend and contracts
You may have heard about IPFS before, the Interplanetary File System. The concept has existed for quite some time now, but with IPFS you'll get a more reliable data storage, thanks to their internal use of blockchain technology. Filecoin is a new system that is incentivizing storage for IPFS...
Downsizing contracts to fight the contract size limit
What can you do to prevent your contracts from getting too large?
Why is there a limit? On November 22, 2016 the Spurious Dragon hard-fork introduced EIP-170 which added a smart contract size limit of 24.576 kb. For you as a Solidity developer this means when you add more and more functionality to your contract, at some point you will reach the limit and when...
Using EXTCODEHASH to secure your systems
How to safely integrate anyone's smart contract
What is the EXTCODEHASH? The EVM opcode EXTCODEHASH was added on February 28, 2019 via EIP-1052. Not only does it help to reduce external function calls for compiled Solidity contracts, it also adds additional functionality. It gives you the hash of the code from an address. Since only contract...
Using the new Uniswap v2 in your contracts
What's new in Uniswap v2 and how to integrate Uniswap v2
Note : For Uniswap 3 check out the tutorial here. What is UniSwap? If you're not familiar with Uniswap yet, it's a fully decentralized protocol for automated liquidity provision on Ethereum. An easier-to-understand description would be that it's a decentralized exchange (DEX) relying on external...
Solidity and Truffle Continuous Integration Setup
How to setup Travis or Circle CI for Truffle testing along with useful plugins.
Continuous integration (CI) with Truffle is great for developing once you have a basic set of tests implemented. It allows you to run very long tests, ensure all tests pass before merging a pull request and to keep track of various statistics using additional tools. We will use the Truffle...
Upcoming Devcon 2021 and other events
The Ethereum Foundation just announced the next Devcon in 2021 in Colombia
Biggest virtual hackathon almost finished First of all, the current HackMoney event has come to an end and it has been a massive success. One can only imagine what kind of cool projects people have built in a 30 days hackathon. All final projects can be seen at:...
The Year of the 20: Creating an ERC20 in 2020
How to use the latest and best tools to create an ERC-20 token contract
You know what an ERC-20 is, you probably have created your own versions of it several times (if not, have a look at: ERC-20). But how would you start in 2020 using the latest tools? Let's create a new ERC-2020 token contract with some basic functionality which focuses on simplicity and latest...
How to get a Solidity developer job?
There are many ways to get a Solidity job and it might be easier than you think!
You have mastered the basics of Solidity, created your first few useful projects and now want to get your hands on some real-world projects. Getting a Solidity developer job might be easier than you think. There are generally plenty of options to choose from and often times not a lot of...
Design Pattern Solidity: Mock contracts for testing
Why you should make fun of your contracts
Mock objects are a common design pattern in object-oriented programming. Coming from the old French word 'mocquer' with the meaning of 'making fun of', it evolved to 'imitating something real' which is actually what we are doing in programming. Please only make fun of your smart contracts if you...
Kickstart your Dapp frontend development with create-eth-app
An overview on how to use the app and its features
Last time we looked at the big picture of Solidity and already mentioned the create-eth-app. Now you will find out how to use it, what features are integrated and additional ideas on how to expand on it. Started by Paul Razvan Berg, the founder of sablier, this app will kickstart your frontend...
The big picture of Solidity and Blockchain development in 2020
Overview of the most important technologies, services and tools that you need to know
Now, I do not know about you, but I remember when I first started with Solidity development being very confused by all the tools and services and how they work in connection with one another. If you are like me, this overview will help you understand the big picture of Solidity development. As I...
Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used...
How to setup Solidity Developer Environment on Windows
What you need to know about developing on Windows
Using Windows for development, especially for Solidity development, can be a pain sometimes, but it does not have to be. Once you have configured your environment properly, it can actually be extremely efficient and Windows is a very, very stable OS, so your overall experience can be amazing. The...
Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
Update 2023 : Ethereum transitioned to Proof of Stake! If you are interested in the randomness there, you can now use the updated info over at https://soliditydeveloper.com/prevrandao. When we talk about randomness and blockchain, these are really two problems: How to generate randomness in smart...