math-q4.js

const f = [new BigNumber("1"), new BigNumber("1")];
let i = 2;

function factorial(n) {
  if (typeof f[n] != 'undefined')
    return f[n];
  var result = f[i - 1];
  for (; i <= n; i++)
    f[i] = result = result.multiply(i.toString());
  return result;
}

let res;
for (var n = 0; n < 1000; n++) {
  if ((factorial(n) / factorial(n - (n - 2))) == factorial(n + 1) / (factorial(3) * factorial(n - 1))) {
    res = n
    break
  }
}
console.log(res)