I'm new using Ethereum. I set up a testchain to play around and have deployed some contracts there. My goal now would be to do something simple, like getting the balance of some of my testchain accounts in a website.

I found this code on the Ethereum documentation here (slightly adapted to get the scripts for web3 and bignumber):


<script type="text/javascript" src="bignumber.js/bignumber.min.js"></script>
<script type="text/javascript" src="dist/web3-light.js"></script>
<script type="text/javascript">

    var Web3 = require('web3');
    var web3 = new Web3();
    web3.setProvider(new web3.providers.HttpProvider());
    function watchBalance() {
        var coinbase = web3.eth.coinbase;
        var originalBalance = web3.eth.getBalance(coinbase).toNumber();
        document.getElementById('coinbase').innerText = 'coinbase: ' + coinbase;
        document.getElementById('original').innerText = ' original balance: ' + originalBalance + '    watching...';
        web3.eth.filter('latest').watch(function() {
            var currentBalance = web3.eth.getBalance(coinbase).toNumber();
            document.getElementById("current").innerText = 'current: ' + currentBalance;
            document.getElementById("diff").innerText = 'diff:    ' + (currentBalance - originalBalance);
    <h1>coinbase balance</h1>
    <button type="button" onClick="watchBalance();">watch balance</button>
    <div id="coinbase"></div>
    <div id="original"></div>
    <div id="current"></div>
    <div id="diff"></div>

I have been running the following command to start geth:

geth --identity "TestNode" --genesis testchain/customGenesis.json --rpc --rpcport "8123" --rpccorsdomain "*" --datadir testchain --port "30303" --nodiscover --ipcapi "admin,db,eth,debug,miner,net,shh,txpool,personal,web3" --rpcapi "db,eth,net,web3" --autodag --networkid 1378 --nat "any" console

I thought that by adding

web3.setProvider(new web3.providers.HttpProvider('http://localhost:8123');

to the HTML file above it could work but if I navigate to http://localhost:8123 I get the following error:

{"id":-1,"jsonrpc":"2.0","error":{"code":-32600,"message":"Could not decode request"}}

I realize my question is probably related to some basic error but I have been reading and haven't managed to figure it out. Any answer or pointers to guides would be greatly appreciated!

1 Answer 1


The error code is explained as

-32600 Invalid Request The JSON sent is not a valid Request object.

One possible solution is to run geth in minimal commands e.g.:

geth --genesis testchain/customGenesis.json --rpc --rpcport "8123" --rpccorsdomain "*" --datadir testchain --nodiscover --networkid 1378  console

Connect with

web3.setProvider(new web3.providers.HttpProvider('http://localhost:8123');

If this doesn't work you can try --rpcaddr ""

  • You put --rpcport "8123" and web3.setProvider(new web3.providers.HttpProvider('http://localhost:8545');. Shouldn't they both be 8123? That is how I did it and it worked! Mar 28, 2016 at 18:54
  • Any port will work, both needs to be same. Fixed the typo.
    – niksmac
    Mar 29, 2016 at 1:48

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