Contract Source Code:

pragma solidity =0.6.6;

contract MockToken {
    string public constant name = "Mock Token";
    string public constant symbol = "MTK";
    uint8 public constant decimals = 18;
    uint256 public totalSupply;
    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;

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

    constructor(uint256 _initialSupply) public {
        totalSupply = _initialSupply;
        balanceOf[msg.sender] = _initialSupply;
        emit Transfer(address(0), msg.sender, _initialSupply);
    }

    function transfer(address to, uint256 value) external returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    function approve(address spender, uint256 value) external returns (bool) {
        allowance[msg.sender][spender] = value;
        emit Approval(msg.sender, spender, value);
        return true;
    }

    function transferFrom(address from, address to, uint256 value) external returns (bool) {
        require(allowance[from][msg.sender] >= value, "ERC20: insufficient allowance");
        allowance[from][msg.sender] -= value;
        _transfer(from, to, value);
        return true;
    }

    function _transfer(address from, address to, uint256 value) internal {
        require(from != address(0), "ERC20: transfer from the zero address");
        require(to != address(0), "ERC20: transfer to the zero address");
        require(balanceOf[from] >= value, "ERC20: transfer amount exceeds balance");

        balanceOf[from] -= value;
        balanceOf[to] += value;
        emit Transfer(from, to, value);
    }
}