1. 扁平数组转成tree结构(来源: StackOverflow的印度老哥写的)

// Data Set
// One top level comment 
var comments = [{
    id: 1,
    parent_id: null
}, {
    id: 2,
    parent_id: 1
}, {
    id: 3,
    parent_id: 1
}, {
    id: 4,
    parent_id: 2
}, {
    id: 5,
    parent_id: 4
}];

const nest = (items, id = null, link = 'parent_id') =>
  items
    .filter(item => item[link] === id)
    .map(item => ({ ...item, children: nest(items, item.id) }));

nest(comments);