const ethers = require('ethers');
const BigNumber = require('bignumber.js');

const provider = new ethers.providers.JsonRpcProvider('https://ropsten.infura.io/v2/INFURA_PROJECT_ID');
const addressFrom = 'SENDER_ADDRESS';
const privateKey = 'SENDER_PRIVATE_KEY';
const wallet = new ethers.Wallet(privateKey, provider);

const addressTo = 'RECEIVER_ADDRESS';

// token details
const tokenAddress = 'TOKEN_ADDRESS';
const amount = 0.5;
const tokenDecimal = 18; // token decimal

/** Create token transfer value as (10 ** token_decimal) * user_supplied_value
 * @dev BigNumber instead of BN to handle decimal user_supplied_value
 */
const base = new BigNumber(10);
const valueToTransfer = base.pow(tokenDecimal)
  .times(amount);
/* Create token transfer ABI encoded data
 * `transfer()` method signature at https://eips.ethereum.org/EIPS/eip-20
 * ABI encoding ruleset at https://solidity.readthedocs.io/en/develop/abi-spec.html
 */
var abi = [{
  "constant": false,
  "inputs": [{
    "name": "_to",
    "type": "address"
  }, {
    "name": "_value",
    "type": "uint256"
  }],
  "name": "transfer",
  "outputs": [{
    "name": "",
    "type": "bool"
  }],
  "payable": false,
  "stateMutability": "nonpayable",
  "type": "function"
}];
const iface = new ethers.utils.Interface(abi);
const rawData = iface.functions.transfer.encode([addressTo, valueToTransfer.toString()]);

provider.getTransactionCount(addressFrom)
  .then((txCount) => {
    // construct the transaction data
    const txData = {
       nonce: txCount,
       gasLimit: 250000,
       gasPrice: 10e9, // 10 Gwei
       to: tokenAddress, // token contract address
       value: ethers.constants.HexZero, // no ether value
       data: rawData,
     }
    const serializedTx = ethers.utils.serializeTransaction(txData);
    //console.log('serialized Tx: ', serializedTx);
    //console.log('parsed serialized tx: ', ethers.utils.parseTransaction(serializedTx));
    // parse unsigned serialized transaction and sign it
    wallet.sign(ethers.utils.parseTransaction(serializedTx))
      .then((signedSerializedTx) => {
        //console.log('signed serialized tx: ', signedSerializedTx);
        // broadcast signed serialized tx
        provider.sendTransaction(signedSerializedTx)
          .then((response) => {
            console.log('tx response: ', response);
          });
      });
  });