[Ethereum] How to convert big number to normal number

I believe you're having an issue in the Promise. The first call has not resolved by the time you make the second call, and so roundNumber is not defined.

Try something like this:

var roundNumber;`
contractInstance.roundNumber.call().then(function (result) {
  roundNumber = Number(result);
  console.log(result + ' is the result'); // returns "0 is the result"
  return roundNumber;
}).then( function (roundNumber) {
  return contractInstance.lotteries.call(roundNumber);
}).then( function (result) {
  console.log(result[0].toString());
  console.log(result[1].toString());
})

I'm not that familiar with promises so my syntax may be incorrect, but I believe that this is the core of your issue.

Contract function calls in Truffle return promises so you have to add a callback to get the response. If getAge() is declared as either constant, pure or view in the Solidity contract you can do instance.getAge().then(age => age.toNumber()) if it doesn't have any of those modifiers then you need to do instance.getAge.call().then(age => age.toNumber())