数组转树

            // 将 input 转成output的形式
            let input = [
                {
                    id: 2,
                    val: '班级1',
                    parentId: 1,
                },
                {
                    id: 3,
                    val: '班级2',
                    parentId: 1,
                },
                {
                    id: 1,
                    val: '学校',
                    parentId: null,
                },
                {
                    id: 4,
                    val: '学生1',
                    parentId: 2,
                },
                {
                    id: 5,
                    val: '学生2',
                    parentId: 2,
                },
                {
                    id: 6,
                    val: '学生3',
                    parentId: 3,
                },
            ];
            function getTree(array) {
                // 找到parentId为null的对象
                let first_index = array.findIndex(item => {
                    if (item.parentId == null) {
                        return true;
                    }
                });
                // 取出对应对象并保存
                let first = array[first_index];
                array.splice(first_index, 1);
                // 新建对象
                let tree = {
                    id: first.id,
                    val: first.val,
                    children: array.length > 0 ? toTree(first.id, array) : [],
                };
                return tree;
            }
            function toTree(id, array) {
                // 新建空数组
                let l = array.length;
                let children = [];
                // 遍历传入的数组
                for (let index = 0; index < l; index++) {
                    let node = array[index];
                    // 如果parentId等于传入的父id则添加对应对象到新建的数组中
                    if (node.parentId == id) {
                        children.push({
                            id: node.id,
                            val: node.val,
                            children: toTree(node.id, array),
                        });
                    }
                }
                return children;
            }
            let output = getTree(input);
            console.log(output);

结果：

output = {
    id: 1,
    val: '学校',
    children: [{
        id: 2,
        val: '班级1',
        children: [
            {
                id: 4,
                val: '学生1',
                children: []
            },
            {
                id: 5,
                val: '学生2',
                children: []
            }
        ]
    }, {
        id: 3,
        val: '班级2',
        children: [{
            id: 6,
            val: '学生3',
            children: []
        }]
    }]
}