Simple Debug during tests

Question

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) {
            console.log(event);
        })
            .on('data', function (event) {
                console.log("OK");
                console.log(event); 
            })
            .on('changed', function (event) {
                console.log("OK");
            })
            .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?

Thanks

UPDATE

My web3 istance

helpers.js

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)

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();
            console.log(livello.events.foo.returnValues);
}

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