combination.js

// in:   [['xxx'], ['yyy', 'zzz'], ...]
// out: [['xxx', 'yyy', ...], ['xxx', 'zzz', ...]]
function calcCombination(options) {
      if (_.isEmpty(options)) {
        return [[]]
      }
      let [firstOpt, ...rest] = options
      let remainCombinations = calcCombination(rest)
      return _.flatMap(remainCombinations, rc => {
        return firstOpt.map(o1 => [o1, ...rc])
      })
    }