nodejs-console.sh

# After deploying the contract

mkdir sgdt
cd sgdt
yarn add web3 # or npm install web3
node

# Now you can run web3-commands.js

SGDT.sol

pragma solidity ^0.4.2;

contract SGDT {
    /* Public variables of the token */
    string public name = 'Singapore Dollar Token';
    string public symbol = 'SGDT';
    uint8 public decimals = 2;
    uint public totalSupply = 0;
    address public authority;
    
    /* This creates an array with all balances */
    mapping (address => uint) public balanceOf;
    mapping (address => uint) public issuedBy;
    mapping (address => bool) public isIssuer;

    /* This generates a public event on the blockchain that will notify clients */
    event Transfer(address from, address to, uint value);
    event TokenIssued(address by);
    
    modifier onlyIssuer {
        if (isIssuer[msg.sender] != true) {
            throw;
        }
        _;
    }
    
    modifier onlyAuthority {
        if (msg.sender != authority) {
            throw;
        }
        _;
    }

    /* Constructor: initializes contract with initial supply tokens to the creator of the contract */
    function SGDT() {
        authority = msg.sender;
        isIssuer[msg.sender] = true;
    }
    
    function addIssuer(address _newIssuer) public onlyAuthority {
        isIssuer[_newIssuer] = true;
    }
    
    // TODO: removeIssuer()
    
    /* Issue SGT */
    function issue(uint _value) public onlyIssuer  {
        issuedBy[msg.sender] += _value;
        balanceOf[msg.sender] += _value;
        totalSupply += _value;
        TokenIssued(msg.sender);
    }

    /** Remove SGDT from blockchain */
    function redeem(uint _value) public onlyIssuer {
        if (balanceOf[msg.sender] < _value || issuedBy[msg.sender] < _value) {
            throw;                                  
        }

        issuedBy[msg.sender] -= _value;
        balanceOf[msg.sender] -= _value;
        totalSupply -= _value;
        TokenIssued(msg.sender);
    }

    /* Send coins */
    function transfer(address _to, uint _value) public {
        if (balanceOf[msg.sender] < _value) {
            throw;                                   // Check if the sender has enough
        }
        balanceOf[msg.sender] -= _value;             // Subtract from the sender
        balanceOf[_to] += _value;                    // Add the same to the recipient
        Transfer(msg.sender, _to, _value);           // Notify anyone listening that this transfer took place
    }

    /* This unnamed function is called whenever someone tries to send ether to it */
    function () {
        throw;     // Prevents accidental sending of ether
    }
}

web3-commands.js

const Web3 = require('web3');
const web3 = new Web3(
  new Web3.providers.HttpProvider(
    'http://localhost:8545'
  )
);

web3;
web3.eth;
web3.eth.accounts;

// Other interesting commands
web3.eth.blockNumber;
web3.eth.getBalance(web3.eth.accounts[0]).toNumber();
web3.eth.getBalance(web3.eth.accounts[3]).toNumber(); // 1 ether = 1e18 wei
web3.fromWei(web3.eth.getBalance(web3.eth.accounts[3]), 'ether').toNumber();

web3.eth.gasPrice; 
web3.fromWei(web3.eth.gasPrice, 'szabo').toNumber(); // 0.02 szabo or 20 Gwei (20e9 wei)
web3.fromWei(web3.eth.gasPrice, 'ether').toNumber(); // 2e-8 ether

let address = 'ADDRESS';
let abi = [{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_newIssuer","type":"address"}],"name":"addIssuer","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint8"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"issuedBy","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"isIssuer","outputs":[{"name":"","type":"bool"}],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[],"payable":false,"type":"function"},{"constant":true,"inputs":[],"name":"authority","outputs":[{"name":"","type":"address"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_value","type":"uint256"}],"name":"issue","outputs":[],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"_value","type":"uint256"}],"name":"redeem","outputs":[],"payable":false,"type":"function"},{"inputs":[],"type":"constructor"},{"payable":false,"type":"fallback"},{"anonymous":false,"inputs":[{"indexed":false,"name":"from","type":"address"},{"indexed":false,"name":"to","type":"address"},{"indexed":false,"name":"value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"by","type":"address"}],"name":"TokenIssued","type":"event"}];
let SGDT = web3.eth.contract(abi).at(address);

SGDT;
SGDT.authority(); // Read some static values
SGDT.totalSupply(); // BigNumber.js Object
SGDT.totalSupply.toNumber();

let authority = SGDT.authority();
SGDT.isIssuer(web3.eth.accounts[1]); // false
SGDT.addIssuer(web3.eth.accounts[1], {
  from: authority,
  gas: 1000000
});
SGDT.isIssuer(web3.eth.accounts[1]); // true

// If you would like to know how much gas it costs, without committing yet
SGDT.addIssuer.estimateGas(web3.eth.accounts[1], {
  from: authority,
  gas: 1000000
});

SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 0
SGDT.issue(1000, {
  from: web3.eth.accounts[1],
  gas: 1000000
});
SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 1000 ($10.00)


SGDT.balanceOf(web3.eth.accounts[2]).toNumber(); // 0
SGDT.transfer(web3.eth.accounts[2], 499, {
  from: web3.eth.accounts[1],
  gas: 1000000,
});                                       
SGDT.balanceOf(web3.eth.accounts[2]).toNumber(); // 499 ($4.99)
SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 501 ($5.01)