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Token tutorial

A tutorial on how to build an ERC-20 compatible smart contract in Acala EVM+.

Table of contents

About

This is an example that builds upon the echo example. Echo was a simple example on building an intreactable state changing smart contract. Token is an example of ERC20 token implementation in Acala EVM+. We won't be building an administrated or upgradeable token and we will use OpenZeppelin ERC20 implementation. For the setup and naming, replace the echo with token. Let's jump into it!
NOTE: You can refer to the complete code of this tutorial at https://github.com/AcalaNetwork/hardhat-tutorials/tree/master/token​

Smart contract

In this tutorial we will be adding a simple smart contract that imports the ERC20 smart contract from openzeppelin/contracts and has a constructor that sets the initial balance of the sender (which also represents the total supply of the token) as well as the name of the token and its abbreviation:
To be able to import the ERC20 from OpenZeppelin, we have to add @openzeppelin/contracts as a development dependency:
yarn add --dev @openzeppelin/contracts
Now that we have added the @openzeppelin/contracts dependency, we can focus on building our smart contract. Your empty smart contract should look like this:
pragma solidity =0.8.9;
​
contract Token{
}
Import of the ERC20 from @openzeppelin/contracts is done between the pragma definition and the start of the contract block:
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
As we now have access to ERC20.sol from @openzeppelin/contracts, we can set the inheritance of our Token contract:
contract Token is ERC20 {
As the ERC20 already has the full fungible token standard implementation, we only have to add a constructor() function that sets all of the values:
constructor(uint256 _initialBalance) ERC20("Token", "TKN") public {
_mint(msg.sender, _initialBalance);
}
We pass the initial balance of the token as _intialBalance and it will be assigned to the account that initiates the deployment of the smart contract. We also pass Token as the name of our example token and TKN for its abbreviation to the ERC20 contract that our Token contract is inheriting.
This concludes our Token smart contract.
Your contracts/Echo.sol should look like this:
pragma solidity =0.8.9;
​
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
​
contract Token is ERC20 {
constructor(uint256 _initialBalance) ERC20("Token", "TKN") public {
_mint(msg.sender, _initialBalance);
}
}
As the Token smart contract is ready to be compiled, we can use the yarn build command (like we did in the hello-world) to compile the smart contract, which will create the artifacts directory and contain the compiled smart contract.

Test

Your test file should be called Token.js and the empty test along with the import statements should look like this:
const { expect } = require("chai");
const { ContractFactory } = require("ethers");
​
const TokenContract = require("../artifacts/contracts/Token.sol/Token.json");
const NULL_ADDRESS = "0x0000000000000000000000000000000000000000";
​
describe("Token contract", function () {
​
});
To prepare for the testing, we have to define global variables, Token, instance, deployer, user, deployerAddress and userAddress. The Token will be used to store the Token contract factory and the instance will store the deployed Token smart contract. Both deployer and user will store Signers. The deployer is the account used to deploy the smart contract (and the one that will receive the initialBalance). The user is the account we will be using to transfer the tokens to and check the allowance operation. deployerAddress and userAddress hold the addresses of the deployer and user respectively. They will be used to avoid repetitiveness in our tests. Let's assign them values in the beforeEach action:
let Token;
let instance;
let deployer;
let user;
let deployerAddress;
let userAddress;
​
beforeEach(async function () {
[deployer, user] = await ethers.getSigners();
deployerAddress = await deployer.getAddress();
userAddress = await user.getAddress();
Token = new ContractFactory(TokenContract.abi, TokenContract.bytecode, deployer);
instance = await Token.deploy(1234567890);
});
Our test will be split into two sections, Deployment and Operation:
describe("Deployment", function () {
​
});
​
describe("Operation", function () {
});
Deployment block contains the examples that validate the expected initial state of the smart contract:
  1. 1.
    The name should equal Token.
  2. 2.
    The symbol should equal TKN.
  3. 3.
    The total supply of the smart contract should equal 1234567890.
  4. 4.
    The initial balance of the deployer account should equal 1234567890.
  5. 5.
    The user account should have 0 balance.
  6. 6.
    The allowances should be set to 0 when the smart contract is deployed.
it("should set the correct token name", async function () {
expect(await instance.name()).to.equal("Token");
});
​
it("should set the correct token symbol", async function () {
expect(await instance.symbol()).to.equal("TKN");
});
​
it("should set the correct total supply", async function () {
expect(await instance.totalSupply()).to.equal(1234567890);
});
​
it("should assign the initial balance to the deployer", async function () {
expect(await instance.balanceOf(deployerAddress)).to.equal(1234567890);
});
​
it("should not assign value to a random address upon deployment", async function () {
expect(await instance.balanceOf(userAddress)).to.equal(0);
});
​
it("should not assign allowance upond deployment", async function () {
expect(await instance.allowance(deployerAddress, userAddress)).to.equal(0);
expect(await instance.allowance(userAddress, deployerAddress)).to.equal(0);
});
In the Operation describe block we first need to increase the timeout to 50000ms, so that the RPC adapter has enough time to return the required information:
The Operation block in itself is separated into two describe blocks, which are separated in itself:
  1. 1.
    Transfer: Contains the test cases to validate the transaction functionality of ERC20 tokens:
  • transfer(): Validates the correct operation of the transfer() function.
  1. 1.
    Allowances: Contains the test cases to validate the allowances and connected functionality of ERC20 tokens:
  • approve(): Validates the correct operation of the approve() function.
  • increaseAllowance(): Validates the correct operation of the increaseAllowance() function.
  • decreaseAllowance(): Validates the correct operation of the decreaseAllowance() function.
  • transferFrom(): Validates the correct operation of the transferFrom() function.
The contents of the Operation block should look like this:
describe("Transfer", function () {
describe("transfer()", function () {
});
});
​
describe("Allowances", function () {
describe("approve()", function () {
});
​
describe("increaseAllowance()", function () {
​
});
​
describe("decreaseAllowance()", function() {
});
​
describe("transferFrom()", function () {
});
});
The transfer() example validates the following:
  1. 1.
    When transferring tokens, the balances should be updated.
  2. 2.
    Transfer event should be emitted.
  3. 3.
    Transfers to 0x0 address should be reverted.
  4. 4.
    Trying to transfer more than own balance should be reverted.
These examples should look like this:
it("should change the balance of the sender and receiver when transferring token", async function () {
const initialDeployerBalance = await instance.balanceOf(deployerAddress);
const initialUserBalance = await instance.balanceOf(userAddress);
​
await instance.connect(deployer).transfer(userAddress, 500);
​
const finalDeployerBalance = await instance.balanceOf(deployerAddress);
const finalUserBalance = await instance.balanceOf(userAddress);
​
expect(initialDeployerBalance - 500).to.equal(finalDeployerBalance);
expect(initialUserBalance + 500).to.equal(finalUserBalance);
});
​
it("should emit a Transfer event when transfering the token", async function () {
await expect(instance.connect(deployer).transfer(userAddress, 100)).to
.emit(instance, "Transfer")
.withArgs(deployerAddress, userAddress, 100);
});
​
it("should revert the transfer to a 0x0 address", async function () {
await expect(instance.connect(deployer).transfer(NULL_ADDRESS, 100)).to
.be.revertedWith("ERC20: transfer to the zero address");
});
​
it("should revert if trying to transfer amount bigger than balance", async function () {
await expect(instance.connect(deployer).transfer(userAddress, 12345678900)).to
.be.revertedWith("ERC20: transfer amount exceeds balance");
});
The approve() example validates the following:
  1. 1.
    Allowance should be granted for less funds than the owner has.
  2. 2.
    Allowance should be granted for more funds than the owner has.
  3. 3.
    Approval event should be emitted when giving allowance.
  4. 4.
    Call should be reverted if the address receiving the allowance is 0x0.
These examples should look like this:
it("should grant allowance when the caller has enough funds", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
expect(await instance.allowance(deployerAddress, userAddress)).to
.equal(100);
});
​
​
it("should grant allowance when the caller has less funds than the ize of the allowance", async function () {
await instance.connect(deployer).approve(userAddress, 12345678900);
​
expect(await instance.allowance(deployerAddress, userAddress)).to
.equal(12345678900);
});
​
it("should emit Approval event when calling approve()", async function () {
await expect(instance.connect(deployer).approve(userAddress, 100)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 100);
});
​
it("should revert when trying to give allowance to 0x0 address", async function () {
await expect(instance.connect(deployer).approve(NULL_ADDRESS, 100)).to
.be.revertedWith("ERC20: approve to the zero address");
});
The increaseAllowance() example validates the following:
  1. 1.
    Owner should be able to increase allowance to an amount smaller that the total funds that they have.
  2. 2.
    Owner should be able to increase the balance to more than the amount of total funds that they have.
  3. 3.
    Approval event should be emitted.
  4. 4.
    The function can be called even if there was no preexisting allowance.
These examples should look like this:
it("should allow to increase allowance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
const initialAllowance = await instance.allowance(deployerAddress, userAddress);
​
await instance.connect(deployer).increaseAllowance(userAddress, 50);
​
const finalAllowance = await instance.allowance(deployerAddress, userAddress);
​
expect(finalAllowance - initialAllowance).to.equal(50);
});
​
it("should allow to increase allowance above the balance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
const initialAllowance = await instance.allowance(deployerAddress, userAddress);
​
await instance.connect(deployer).increaseAllowance(userAddress, 1234567890);
​
const finalAllowance = await instance.allowance(deployerAddress, userAddress);
​
expect(finalAllowance - initialAllowance).to.equal(1234567890);
});
​
it("should emit Approval event", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).increaseAllowance(userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 140);
});
​
it("should allow to increase allowance even if none was given before", async function () {
await instance.connect(deployer).increaseAllowance(userAddress, 1234567890);
​
const allowance = await instance.allowance(deployerAddress, userAddress);
​
expect(allowance).to.equal(1234567890);
});
The decreaseAllowance() example validates the following:
  1. 1.
    Owner should be able to decrease allowance.
  2. 2.
    Approval event should be emitted.
  3. 3.
    Call should be reverted when trying to decrease the allowance below 0.
These examples should look like this:
We can now add the following test cases to our describe block:
it("should decrease the allowance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await instance.connect(deployer).decreaseAllowance(userAddress, 40);
​
const allowance = await instance.allowance(deployerAddress, userAddress);
​
expect(allowance).to.equal(60);
});
​
it("should emit Approval event", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).decreaseAllowance(userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 60);
});
​
it("should revert when tyring to decrease the allowance below 0", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).decreaseAllowance(userAddress, 1000)).to
.be.revertedWith("ERC20: decreased allowance below zero");
});
The transferFrom() example validates the following:
  1. 1.
    Should allow transfer when allowance is given.
  2. 2.
    Transfer event should be emitted.
  3. 3.
    Approval event should be emitted.
  4. 4.
    Should update allowance.
  5. 5.
    Should revert if trying to transfer more than allowance.
  6. 6.
    Should revert when trying to transfer to 0x0 address.
  7. 7.
    Should revert when owner doesn't have enough funds.
  8. 8.
    Should revert when no allowance was given.
These examples should look like this:
it("should allow to transfer tokens when allowance is given", async function () {
​
await instance.connect(deployer).approve(userAddress, 100);
​
const initialBalance = await instance.balanceOf(userAddress);
​
await instance.connect(user).transferFrom(deployerAddress, userAddress, 50);
​
const finalBalance = await instance.balanceOf(userAddress);
​
expect(initialBalance + 50).to.equal(finalBalance);
});
​
it("should emit Transfer event when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 40)).to
.emit(instance, "Transfer")
.withArgs(deployerAddress, userAddress, 40);
});
​
it("should emit Approval event when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 60);
});
​
it("should update the allowance when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await instance.connect(user).transferFrom(deployerAddress, userAddress, 40);
​
expect(await instance.allowance(deployerAddress, userAddress)).to.equal(60);
});
​
it("should revert when tring to transfer more than allowed amount", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 1000)).to
.be.revertedWith("ERC20: transfer amount exceeds allowance");
});
​
it("should revert when transfering to 0x0 address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(user).transferFrom(deployerAddress, NULL_ADDRESS, 50)).to
.be.revertedWith("ERC20: transfer to the zero address");
});
​
it("should revert when owner doesn't have enough funds", async function () {
await instance.connect(deployer).approve(userAddress, 12345678900);
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 12345678900)).to
.be.revertedWith("ERC20: transfer amount exceeds balance");
});
​
it("should revert when trying to transfer from without being given allowance", async function () {
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 10)).to
.be.revertedWith("ERC20: transfer amount exceeds allowance");
});
With that, our test is ready to be run.
Your test/Token.js should look like this:
const { expect } = require("chai");
const { ContractFactory } = require("ethers");
​
const TokenContract = require("../artifacts/contracts/Token.sol/Token.json");
const NULL_ADDRESS = "0x0000000000000000000000000000000000000000";
​
describe("Token contract", function () {
let Token;
let instance;
let deployer;
let user;
let deployerAddress;
let userAddress;
​
beforeEach(async function () {
[deployer, user] = await ethers.getSigners();
deployerAddress = await deployer.getAddress();
userAddress = await user.getAddress();
Token = new ContractFactory(TokenContract.abi, TokenContract.bytecode, deployer);
instance = await Token.deploy(1234567890);
});
​
describe("Deployment", function () {
it("should set the correct token name", async function () {
expect(await instance.name()).to.equal("Token");
});
​
it("should set the correct token symbol", async function () {
expect(await instance.symbol()).to.equal("TKN");
});
​
it("should set the correct total supply", async function () {
expect(await instance.totalSupply()).to.equal(1234567890);
});
​
it("should assign the initial balance to the deployer", async function () {
expect(await instance.balanceOf(deployerAddress)).to.equal(1234567890);
});
​
it("should not assign value to a random address upon deployment", async function () {
expect(await instance.balanceOf(userAddress)).to.equal(0);
});
​
it("should not assign allowance upon deployment", async function () {
expect(await instance.allowance(deployerAddress, userAddress)).to.equal(0);
expect(await instance.allowance(userAddress, deployerAddress)).to.equal(0);
});
});
​
describe("Operation", function () {
this.timeout(50000);
​
describe("Transfer", function () {
describe("transfer()", function () {
it("should change the balance of the sender and receiver when transferring token", async function () {
const initialDeployerBalance = await instance.balanceOf(deployerAddress);
const initialUserBalance = await instance.balanceOf(userAddress);
​
await instance.connect(deployer).transfer(userAddress, 500);
​
const finalDeployerBalance = await instance.balanceOf(deployerAddress);
const finalUserBalance = await instance.balanceOf(userAddress);
​
expect(initialDeployerBalance - 500).to.equal(finalDeployerBalance);
expect(initialUserBalance + 500).to.equal(finalUserBalance);
});
​
it("should emit a Transfer event when transfering the token", async function () {
await expect(instance.connect(deployer).transfer(userAddress, 100)).to
.emit(instance, "Transfer")
.withArgs(deployerAddress, userAddress, 100);
});
​
it("should revert the transfer to a 0x0 address", async function () {
await expect(instance.connect(deployer).transfer(NULL_ADDRESS, 100)).to
.be.revertedWith("ERC20: transfer to the zero address");
});
​
it("should revert if trying to transfer amount bigger than balance", async function () {
await expect(instance.connect(deployer).transfer(userAddress, 12345678900)).to
.be.revertedWith("ERC20: transfer amount exceeds balance");
});
});
});
​
describe("Allowances", function () {
describe("approve()", function () {
it("should grant allowance when the caller has enough funds", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
expect(await instance.allowance(deployerAddress, userAddress)).to
.equal(100);
});
​
​
it("should grant allowance when the caller has less funds than the ize of the allowance", async function () {
await instance.connect(deployer).approve(userAddress, 12345678900);
​
expect(await instance.allowance(deployerAddress, userAddress)).to
.equal(12345678900);
});
​
it("should emit Approval event when calling approve()", async function () {
await expect(instance.connect(deployer).approve(userAddress, 100)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 100);
});
​
it("should revert when trying to give allowance to 0x0 address", async function () {
await expect(instance.connect(deployer).approve(NULL_ADDRESS, 100)).to
.be.revertedWith("ERC20: approve to the zero address");
});
});
​
describe("increaseAllowance()", function () {
it("should allow to increase allowance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
const initialAllowance = await instance.allowance(deployerAddress, userAddress);
​
await instance.connect(deployer).increaseAllowance(userAddress, 50);
​
const finalAllowance = await instance.allowance(deployerAddress, userAddress);
​
expect(finalAllowance - initialAllowance).to.equal(50);
});
​
it("should allow to increase allowance above the balance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
const initialAllowance = await instance.allowance(deployerAddress, userAddress);
​
await instance.connect(deployer).increaseAllowance(userAddress, 1234567890);
​
const finalAllowance = await instance.allowance(deployerAddress, userAddress);
​
expect(finalAllowance - initialAllowance).to.equal(1234567890);
});
​
it("should emit Approval event", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).increaseAllowance(userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 140);
});
​
it("should allow to increase allowance even if none was given before", async function () {
await instance.connect(deployer).increaseAllowance(userAddress, 1234567890);
​
const allowance = await instance.allowance(deployerAddress, userAddress);
​
expect(allowance).to.equal(1234567890);
});
});
​
describe("decreaseAllowance()", function() {
it("should decrease the allowance", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await instance.connect(deployer).decreaseAllowance(userAddress, 40);
​
const allowance = await instance.allowance(deployerAddress, userAddress);
​
expect(allowance).to.equal(60);
});
​
it("should emit Approval event", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).decreaseAllowance(userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 60);
});
​
it("should revert when tyring to decrease the allowance below 0", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(deployer).decreaseAllowance(userAddress, 1000)).to
.be.revertedWith("ERC20: decreased allowance below zero");
});
});
​
describe("transferFrom()", function () {
it("should allow to transfer tokens when allowance is given", async function () {
​
await instance.connect(deployer).approve(userAddress, 100);
​
const initialBalance = await instance.balanceOf(userAddress);
​
await instance.connect(user).transferFrom(deployerAddress, userAddress, 50);
​
const finalBalance = await instance.balanceOf(userAddress);
​
expect(initialBalance + 50).to.equal(finalBalance);
});
​
it("should emit Transfer event when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 40)).to
.emit(instance, "Transfer")
.withArgs(deployerAddress, userAddress, 40);
});
​
it("should emit Approval event when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
​
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 40)).to
.emit(instance, "Approval")
.withArgs(deployerAddress, userAddress, 60);
});
​
it("should update the allowance when transferring from another address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await instance.connect(user).transferFrom(deployerAddress, userAddress, 40);
​
expect(await instance.allowance(deployerAddress, userAddress)).to.equal(60);
});
​
it("should revert when tring to transfer more than allowed amount", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 1000)).to
.be.revertedWith("ERC20: transfer amount exceeds allowance");
});
​
it("should revert when transfering to 0x0 address", async function () {
await instance.connect(deployer).approve(userAddress, 100);
await expect(instance.connect(user).transferFrom(deployerAddress, NULL_ADDRESS, 50)).to
.be.revertedWith("ERC20: transfer to the zero address");
});
​
it("should revert when owner doesn't have enough funds", async function () {
await instance.connect(deployer).approve(userAddress, 12345678900);
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 12345678900)).to
.be.revertedWith("ERC20: transfer amount exceeds balance");
});
​
it("should revert when trying to transfer from without being given allowance", async function () {
await expect(instance.connect(user).transferFrom(deployerAddress, userAddress, 10)).to
.be.revertedWith("ERC20: transfer amount exceeds allowance");
});
});
});
});
});
When you run the test with (for example) yarn test-mandala, your tests should pass with the following output:
yarn test-mandala
​
​
yarn run v1.22.19
$ hardhat test test/Token.js --network mandala
​