问题 js 树状结构

问题：

• 不明白数组怎么就有children字段了，怎么添加的
• 运行顺序是怎么样的

<script>

export default {
  name: 'app',
  data() {
    return {
      treeData: [
        {
          id: '1',
          name: '1',
          fatherId: '0'
        },
        {
          id: '2',
          name: '1-1',
          fatherId: '1'
        },
        {
          id: '3',
          name: '1-2',
          fatherId: '1'
        },
        {
          id: '4',
          name: '1-1-1',
          fatherId: '2'
        },
        {
          id: '5',
          name: '1-1-2',
          fatherId: '2'
        },
        {
          id: '6',
          name: '2',
          fatherId: '0'
        },
        {
          id: '7',
          name: '1-2-1',
          fatherId: '3'
        }
      ]
    }
  },

  methods: {
    getTreeData: function() {
      const map = {}
      const val = []
      this.treeData.forEach((item) => {
        map[item.id] = item
      })
      this.treeData.forEach((item) => {
        const parent = map[item.fatherId]
        console.log(item)
        console.log(parent)
        if (parent) {
          (parent.children || (parent.children = [])).push(item)
        } else {
          val.push(item)
        }
      })
    }
  },
  created() {
    this.getTreeData()
  }
}