js将有父子关系的数据转换成树形结构数据

let data = [{
        id: 1,
        name: 'a',
        parent_id: 0
    }, {
        id: 2,
        name: 'b',
        parent_id: 0
    }, {
        id: 3,
        name: 'a-1',
        parent_id: 1
    }, {
        id: 4,
        name: 'a-2',
        parent_id: 1
    }, {
        id: 5,
        name: 'b-1',
        parent_id: 2
    }, {
        id: 6,
        name: 'b-2',
        parent_id: 2
    }, {
        id: 7,
        name: 'a-1-1',
        parent_id: 3
    }, ];
    let result = data.reduce(function(prev, item) {
        prev[item.parent_id] ? prev[item.parent_id].push(item) : prev[item.parent_id] = [item];
        return prev;
    }, {});
    for (let prop in result) {
        result[prop].forEach(function(item, i) {
            result[item.id] ? item.children = result[item.id] : ''
        });
    }
    result = result[0];
    console.log(JSON.stringify(result))