額，就是不想寫循環(huán)

let json2Obj = json2.reduce((acc, cur) => {

    let childId = cur.childId;

    if (!acc[childId]) {

        acc[childId] = [];

    }

    acc[childId].push(cur);

    return acc;

}, {})

json1.forEach(item => {

    item.childchild = json2Obj[item.childId]

})

let json1Obj = json1.reduce((acc, cur) => {

    let mainId = cur.mainId;

    if (!acc[mainId]) {

        acc[mainId] = [];

    }

    acc[mainId].push(cur);

    return acc;

}, {})

json.forEach(item => {

    item.child = json1Obj[item.mainId];

})

console.log(JSON.stringify(json));

reduce重構(gòu)下：

const toObj = (json, idStr) => json.reduce((acc, cur) => {

    let id = cur[idStr];

    if (!acc[id]) {

        acc[id] = [];

    }

    acc[id].push(cur);

    return acc;

}, {})

const json2Obj = toObj(json2, 'childId');

json1.forEach(item => item.childchild = json2Obj[item.childId])

const json1Obj = toObj(json1, 'mainId');

json.forEach(item => item.child = json1Obj[item.mainId])

console.log(JSON.stringify(json));