How can I test my purchase function on the truffle with chai?

Question

I'm trying to test my token buying function but I encounter an error, o facilitate understanding I will show the code

"Error: Returned error: VM Exception while processing transaction: revert"

my purchase function, In the remix it works perfectly:

uint public buyprice = 50000000000000 wei;
    
function buy() public payable {
            require(msg.value >= buyprice);
            require(_balances[address(this)] >= msg.value.div(buyprice) * 10 ** uint(decimals));
            
            amount_eth += msg.value;
            uint tokens = msg.value.div(buyprice) * 10 ** uint(decimals);
            _balances[msg.sender] += tokens;
            
            
            emit Transfer(address(this), msg.sender, tokens);
        }

I'm trying to test it that way:

it("buy tokens", async () => {
        let instance = this.StudyCoin;
        const weiAmount = new BN(5000000000000000);

        let buyTokens = await instance.buy();
        let balanceOfDeployer = await instance.balanceOf(deployerAccount);

        expect(instance.sendTransaction({from: deployerAccount, value: web3.utils.toWei(0.005, "ether")})).to.be.fulfilled;
        balanceOfDeployer = balanceOfDeployer.add(new BN(100000000000000000000));
        console.log(balanceOfDeployer);
        return expect(instance.balanceOf(deployerAccount)).to.eventually.be.a.bignumber.equal(balanceOfDeployer);

    });

I tried several searches and alternatives but without success. I'm using pragma 0.7.4, can someone give me some idea how to do this test, will open doors for several other tests thanks

Answer 1

Since buy is expecting ethers you should send it in the same transaction.

it("buy tokens", async () => {
    let instance = this.StudyCoin;
    const weiAmount = new BN(5000000000000000);

    let buyTokens = await instance.buy({
        from: deployerAccount,
        value: web3.utils.toWei(0.005, "ether")
    });

    ..
});