Getting an 'invalid opcode' calling a solidty function from test file

Question

im in the process of learning about ethereum and Solidity, and im having a problem calling a payable function from a test file.

This is my contract

pragma solidity >=0.4.22 <0.6.0;
//pragma solidity 0.4.17

contract Lottery {

    address public manager;
    address payable [] public players;

    constructor() public {
        manager = msg.sender;
    }

    function enter() public payable {
        require(msg.value > .01 ether);
        players.push(msg.sender);
    }

    function pickWinner() public payable restricted {
        uint index = random() % players.length;
        address payable winner = players[index];
        winner.transfer(address(this).balance);
        players = new address payable [](0);
    }

    function random() private view returns (uint) {
        return uint( keccak256(abi.encode(block.difficulty, now, players)));
    }

    modifier restricted() {
        require(msg.sender == manager, 'Your are not the manager!');
        _;
    }

    function getPlayers() public view returns (address payable[] memory) {
        return players;
    }
}

and this is my test file

let lottery;
let accounts;

beforeEach(async () => {
  accounts = await web3.eth.getAccounts();
  lottery = await new web3.eth.Contract(abi)
    .deploy({data: bytecode.object})
    .send({from: accounts[0], gas: '1000000'}});
});

describe('Lottery Contract', () => {
  it('deploys contract', () => {
    assert.ok(lottery.options.address);
  });

  it('Allows 1 account to enter', async () => {
    await lottery.methods.enter().send({
      from: accounts[0],
      value: web3.utils.toWei('0.02', 'ether')
    });

    const players = await lottery.methods.getPlayers().call({
      from: accounts[0]
    });

    assert.ok(accounts[0], players[0]);
    assert.ok(1, players.length);
  });


  it('Allows multiple accounts to enter', async () => {
    let conta = 0;
    for(let account of accounts) {
      await lottery.methods.enter().send({
        from: account,
        value: web3.utils.toWei('0.02', 'ether')
      });

      const players = await lottery.methods.getPlayers().call({
        from: account
      });

      assert.equal(account, players[conta]);
      assert.equal(conta + 1, players.length);
      conta++;
    }
  });

  it('Requires a minimun ammount to enter', async () => {
    try{
      await lottery.methods.enter().send({
        from: accounts[0],
        value: 200
      });
      assert(false);
    } catch(error){
      assert(error);
    }
  });

  it('only manager can call pick winner', async () => {
    try{
      await lottery.methods.pickWinner().send({
        from: accounts[1],
      });
      assert(false);
    } catch (error) {
      console.log(error);
      assert(error);
    }
  });

  it('manager can call pick winner', async () => {
      await lottery.methods.pickWinner().send({
        from: accounts[0],
      });
      assert(true);
  });

});

No matter what I try, I'm still getting the "invalid opcode" error when calling lottery.methods.pickWinner()

This are the lib versions I'm using

 "ganache-cli": "^6.7.0",
 "mocha": "^6.2.2",
 "solc": "^0.5.14",
 "truffle-hdwallet-provider": "^1.0.17",
 "web3": "^1.2.4"

The full trace I'm getting is:

o: VM Exception while processing transaction: invalid opcode
      at Function.o.fromResults (node_modules\ganache-cli\build\ganache-core.node.cli.js:10:89727)
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:25:121953
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:25:98498
      at p (node_modules\ganache-cli\build\ganache-core.node.cli.js:25:98156)
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:25:98209
      at t.default (node_modules\ganache-cli\build\ganache-core.node.cli.js:37:544097)
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:25:99122
      at t.n.emit (node_modules\ganache-cli\build\ganache-core.node.cli.js:61:16355)
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:61:654328
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:61:654351
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:37:1093446
      at D:\DEVELOPMENT\repo\study\etherumSolidityCOurse\lottery\node_modules\ganache-cli\build\ganache-core.node.cli.js:2:19209

Already checked Invalid Opcode opcode error Is it normal to have Invalid opcode in a contract? and VM error: invalid opcode error caused by a modifier but im pretty sure its not my case

Answer 1

Your test uses a beforeEach() to set up new, blank contract to play with. That's an agreeable thing so no test is dependent on any other.

The function uses a modulo to pick a row and then forges ahead to work, but if there are players, because enter() wasn't called first, then the modulo will be zero and it will try to access row 0 in players and that doesn't exist.

You can raise a more meaningful response, but the contract is already doing the right thing.

    function pickWinner() public payable restricted {
        require(players.length > 0, "Need at least one player."); // <== this
        uint index = random() % players.length;
        address payable winner = players[index];
        winner.transfer(address(this).balance);
        players = new address payable [](0);
    }

Do a few of these before you try to pick a winner:

      await lottery.methods.enter().send({
        from: account,
        value: web3.utils.toWei('0.02', 'ether')

Hope it helps.