I am fairly new to DAPP and I am trying to follow an online guide building my first smart contract. Here's the source code:


pragma solidity ^0.4.2;

contract Election {

    string public candidateName;

    constructor (string _candidateName) public {
        candidateName = _candidateName;

    function setCandidate (string _name) public {
        candidateName = _name;


I've deployed to to my local Ganache (latest version on windows 10) and tried to interact with it using the Web3.js from here.

Here's my JavaScript code for my client side:

// Initialise web3
if (typeof web3 !== "undefined") {
  web3 = new Web3(web3.currentProvider);
} else {
  web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:7545"));

// Set account
web3.eth.defaultAccount = web3.eth.accounts[0];

// Set contract abi
var contractAbi = [ ... my aib here ... ];

// Set contract address
var contractAddress = "my-contract-address-here";

// Load the contract
var contract = web3.eth.contract(contractAbi).at(contractAddress);

// Display current candidate name
contract.candidateName(function(err, candidateName) {
  if (err) {
    alert("Error: " + err);
  } else {

// Handle candidate name update
$("#updateCandidate").on("click", function(event) {

When I run my app in the browser, I am getting the following error:

web3.min.js:1 Failed to load http://localhost:7545/: Response to preflight request doesn't pass access control check: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'null' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.

Uncaught Error: CONNECTION ERROR: Couldn't connect to node http://localhost:7545. at Object.InvalidConnection (web3.min.js:1) at t.send (web3.min.js:1) at n.send (web3.min.js:1) at l.accounts (web3.min.js:1) at script.js:9 InvalidConnection @ web3.min.js:1 t.send @ web3.min.js:1 n.send @ web3.min.js:1 (anonymous) @ web3.min.js:1 (anonymous) @ script.js:9

Any idea what I might be doing wrong here?

1 Answer 1


I think I've worked out what the problem is.

Since my "client" doesn't technically need a webserver, I ran my html app directly from my desktop, i.e. it was executing from this url in chrome:


After I moved it into a local webserver (e.g. apache) and running it from this url:


It's now working, I am no longer getting CORS errors.

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