I'm new in solidity and I'd like to have a little bit of debug.

this is my simple example (I'm using web3 1.0)

I have a simple function in my contract

function bar(address _address_usr) public  {
        uint res = 5;
        emit foo('test',res);       

And I have test about it

it("...", async () => {

        let livello = await instance.methods.bar(accounts[0]).call();

        instance.events.foo(function (error, event) {
            .on('data', function (event) {
            .on('changed', function (event) {
            .on('error', console.error);    


I'd like to know the result of res. I tried with event but I have this error

err Error: The current provider doesn't support subscriptions: HttpProvider

Can I print some result/var in my console like console.log or something like that?



My web3 istance


const Web3 = require("web3") // import web3 v1.0 constructor

// use globally injected web3 to find the currentProvider and wrap with web3 v1.0
const getWeb3 = () => {
    const myWeb3 = new Web3(web3.currentProvider)
    return myWeb3

// assumes passed-in web3 is v1.0 and creates a function to receive contract name
const getContractInstance = (web3) => (contractName) => {
    const artifact = artifacts.require(contractName) // globally injected artifacts helper
    const deployedAddress = artifact.networks[artifact.network_id].address
    const instance = new web3.eth.Contract(artifact.abi, deployedAddress)
    return instance

module.exports = { getWeb3, getContractInstance }

my test.js

const {getWeb3, getContractInstance} = require("./helpers")
const web3 = getWeb3()
const getInstance = getContractInstance(web3)
  • I recommend looking at how open zeppelin implement tests, they have helper functions to parse events, github.com/OpenZeppelin/openzeppelin-solidity/blob/master/test/…, but honestly I miss the old way they did things :P.
    – GrandFleet
    Commented Nov 19, 2018 at 17:18
  • Try to share the code where you create the web3 instance, I think you might be using websocket functionality and that's where it's failing, to catch the error, do a try catch when generating the instance, to fix the problem don't use ws, or use a web socket compatible node Commented Nov 19, 2018 at 21:30
  • @EnriqueAlcazar I updated my question. the only way to read variable inside my code is events?
    – monkeyUser
    Commented Nov 20, 2018 at 10:16
  • If you want to debug, you can use remix, they have a pretty good debugger, also remember that you can use strings as a second parameter to your reverts, and you can create constant functions (views) to read any parameter in your blockchain Commented Nov 20, 2018 at 10:34
  • Check that web3 you are using is correct, IE: console.log it and look if the params are correct Commented Nov 20, 2018 at 10:35

1 Answer 1


Your error states that you are using HttpProvider for web3 and as per the web3 documentation here, it states:

Object - HttpProvider: The HTTP provider is deprecated, as it won’t work for subscriptions.

and this error is appearing from this code:

// throw error, if provider doesnt support subscriptions
    if(!this.options.requestManager.provider.on) {
        var err2 = new Error('The current provider doesn\'t support subscriptions: '+ this.options.requestManager.provider.constructor.name);
        this.callback(err2, null, this);
        this.emit('error', err2);
        return this;

I think for event subscription you should try to use other alternatives of provider.

UPDATE: To get the value of event parameters you can change the code like this:

   it("...", async () => {

            let livello = await instance.methods.bar(accounts[0]).send();

This will print the event params values. For more, visit: https://web3js.readthedocs.io/en/1.0/web3-eth-contract.html#methods-mymethod-send

Furthermore, if you are looking to print the variables value only, you should rather use view/pure function without events. For more info, visit: https://solidity.readthedocs.io/en/v0.5.0/contracts.html#functions

  • I updated my question
    – monkeyUser
    Commented Nov 20, 2018 at 10:16
  • Are you looking to get the values of event variables from transaction?
    – Aniket
    Commented Nov 20, 2018 at 10:22
  • I'd like to have a method to debug my contract I want to get the val of res for istance, Event is a good approach?
    – monkeyUser
    Commented Nov 20, 2018 at 10:25
  • remove .call() from your function call and you will get it in event params.
    – Aniket
    Commented Nov 20, 2018 at 10:28
  • I have the smae error The current provider doesn't support subscriptions: HttpProvider
    – monkeyUser
    Commented Nov 20, 2018 at 10:34

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