SOURCE

let arr = [
    { id: 2, name: '部门B', parentId: 0 },
    { id: 3, name: '部门C', parentId: 1 },
    { id: 1, name: '部门A', parentId: 2 },

];


function totree(list, parId) {
    let obj = {};
    let result = [];
    //将数组中数据转为键值对结构 (这里的数组和obj会相互引用)
    list.map(el => {
        obj[el.id] = el;
    })
    for (let i = 0, len = list.length; i < len; i++) {
        let id = list[i].parentId;
        if (id == parId) {
            result.push(list[i]);
            continue;
        }
        if (obj[id].children) {
            obj[id].children.push(list[i]);
        } else {
            obj[id].children = [list[i]];
        }
    }
    console.log(result)
    return result;
}
totree(arr, 0)

var arr1 = [{ id: 1, pid: '-1' }, { id: 11, pid: '1' }, { id: 12, pid: '1' }]


function listToTree(list) {
    var map = {}, node, tree = [], i;
    for (i = 0; i < list.length; i++) {
        map[list[i].id] = list[i];
        list[i].children = [];
    }
    for (i = 0; i < list.length; i += 1) {
        node = list[i];
        if (node.pid !== '-1') {
            map[node.pid].children.push(node);
        } else {
            tree.push(node);
        }
    }
    console.log(tree)
    return tree;
}

listToTree(arr1);



function listToTreeWithLevel(list, pid, level) {
    var out = []
    for (var node of list) {
        if (node.pid == pid) {
            node.level = level;
            var children = listToTreeWithLevel(list, node.id, level + 1)
            if (children.length) {
                node.children = children
            }
            out.push(node)
        }
    }

    return out
}

console.log(listToTreeWithLevel(arr1, '-1', 1))