ERC-20 fungible token standard in Blockchain / Solidity - Mini Project: Build & Apply

Create a Simple ERC-20 Fungible Token

📖 Scenario: You want to create your own digital currency on the Ethereum blockchain. This currency will follow the ERC-20 standard, which means it can be easily used by wallets and exchanges.

🎯 Goal: Build a simple ERC-20 token contract that tracks balances, allows transfers, and lets users approve others to spend tokens on their behalf.

📋 What You'll Learn

Create a contract named SimpleToken

Define state variables for name, symbol, decimals, and totalSupply

Create a mapping called balances to track token balances

Create a mapping called allowances to track approved spending

Implement transfer, approve, and transferFrom functions

Emit Transfer and Approval events appropriately

💡 Why This Matters

🌍 Real World

ERC-20 tokens are the most common type of tokens on Ethereum. They represent digital assets like currencies, points, or shares.

💼 Career

Understanding ERC-20 tokens is essential for blockchain developers working on decentralized finance (DeFi), NFTs, and token-based applications.

1

Set up the basic contract and state variables

Write a Solidity contract named SimpleToken. Inside it, declare public variables name as "MyToken", symbol as "MTK", decimals as 18, and totalSupply as 1000000 * 10 ** 18. Also, create a public mapping balances from address to uint256.

Blockchain / Solidity

contract SimpleToken {
    // Your code here
}

Need a hint?

Start by declaring the contract and the variables exactly as named.

2

Initialize the total supply and add allowances mapping

Add a constructor that assigns the entire totalSupply to the contract deployer's balances. Also, declare a public nested mapping allowances from address to address to uint256.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleToken {
    string public name = "MyToken";
    string public symbol = "MTK";
    uint8 public decimals = 18;
    uint256 public totalSupply = 1000000 * 10 ** 18;

    mapping(address => uint256) public balances;
    // Your code here
}

Need a hint?

Use msg.sender to assign the total supply to the deployer.

3

Implement transfer, approve, and transferFrom functions

Write three public functions: transfer that moves tokens from the caller to another address, approve that allows a spender to spend tokens on the caller's behalf, and transferFrom that allows a spender to transfer tokens from an owner to a recipient. Make sure to update balances and allowances accordingly and return bool success.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleToken {
    string public name = "MyToken";
    string public symbol = "MTK";
    uint8 public decimals = 18;
    uint256 public totalSupply = 1000000 * 10 ** 18;

    mapping(address => uint256) public balances;
    mapping(address => mapping(address => uint256)) public allowances;

    constructor() {
        balances[msg.sender] = totalSupply;
    }

    // Your code here
}

Need a hint?

Use require to check balances and allowances. Emit events after state changes.

4

Test the transfer function output

Add a simple test in the contract that transfers 100 tokens from the deployer to address 0x0000000000000000000000000000000000000001 and then print the balance of that address using a public view function. Write a public view function balanceOf that returns the balance of a given address. Then call balanceOf for 0x0000000000000000000000000000000000000001 and print the result.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleToken {
    string public name = "MyToken";
    string public symbol = "MTK";
    uint8 public decimals = 18;
    uint256 public totalSupply = 1000000 * 10 ** 18;

    mapping(address => uint256) public balances;
    mapping(address => mapping(address => uint256)) public allowances;

    constructor() {
        balances[msg.sender] = totalSupply;
    }

    function transfer(address to, uint256 amount) public returns (bool) {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        balances[msg.sender] -= amount;
        balances[to] += amount;
        emit Transfer(msg.sender, to, amount);
        return true;
    }

    function approve(address spender, uint256 amount) public returns (bool) {
        allowances[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(address from, address to, uint256 amount) public returns (bool) {
        require(balances[from] >= amount, "Insufficient balance");
        require(allowances[from][msg.sender] >= amount, "Allowance exceeded");

        balances[from] -= amount;
        balances[to] += amount;
        allowances[from][msg.sender] -= amount;

        emit Transfer(from, to, amount);
        return true;
    }

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    // Your code here
}

Need a hint?

Write a balanceOf function and a testTransfer function that transfers 100 tokens and returns the new balance.