code: function generateLevelMap () {
    LEVELS.forEach(level => {
      const levelIndex = LEVELS.indexOf(level)
      levelMap[level] = {}
      LEVELS.forEach(type => {
        const typeIndex = LEVELS.indexOf(type)
        if (typeIndex <= levelIndex) {
          levelMap[level][type] = true
        }
      })
    })
  }
docs: Generate map for which types of message will be sent in a certain message level
  as the order of LEVELS.