simple ethereum token

ERC20.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity >=0.8.0;

/// @notice Basic ERC20 implementation.
contract ERC20 {
    string public name;
    string public symbol;
    uint8 constant public decimals = 18;
    uint public totalSupply;
    
    mapping(address => uint) public balanceOf;
    mapping(address => mapping(address => uint)) public allowance;
    
    event Transfer(address indexed from, address indexed to, uint amount);
    event Approval(address indexed owner, address indexed spender, uint amount);
    
    constructor(string memory _name, string memory _symbol, uint _totalSupply) {
        name = _name;
        symbol = _symbol;
        totalSupply = _totalSupply;
        balanceOf[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }
    
    function approve(address to, uint amount) external returns (bool) {
        allowance[msg.sender][to] = amount;
        emit Approval(msg.sender, to, amount);
        return true;
    }
    
    function transfer(address to, uint amount) external returns (bool) {
        balanceOf[msg.sender] -= amount;
        balanceOf[to] += amount;
        emit Transfer(msg.sender, to, amount);
        return true;
    }
    
    function transferFrom(address from, address to, uint amount) external returns (bool) {
        if (allowance[from][msg.sender] != type(uint).max) 
            allowance[from][msg.sender] -= amount;
        balanceOf[from] -= amount;
        balanceOf[to] += amount;
        emit Transfer(from, to, amount);
        return true;
    }
}