JSFiddle - React, Tailwind, and code Playground

Babel + JSX

let a = [{
  id: 1,
  nested: [{
    id: 1,
    nestedName: 'NestName1',
    count: 3
  }, {
    id: 2,
    nestedName: 'NestName2',
    count: 4
  }],
  name: 'parent1'
}, {
  id: 2,
  nested: [{
    id: 3,
    nestedName: 'NestName3',
    count: 3
  }, {
    id: 2,
    nestedName: 'NestName2',
    count: 3
  }, {
    id: 4,
    nestedName: 'NestName4',
    count: 3
  }, 
  {
    id: 20,
    nestedName: 'NestName4',
    count: 6
  }, 
  {
    id: 20,
    nestedName: 'NestName4',
    count: 7
  }],
  name: 'parent2'
}, {
  id: 3,
  nested: [{
    id: 5,
    nestedName: 'NestName5',
    count: 3
  }, {
    id: 6,
    nestedName: 'NestName6',
    count: 3
  }, {
    id: 4,
    nestedName: 'NestName4',
    count: 3
  }, 
   {
    id: 30,
    nestedName: 'NestName6',
    count: 5
  }],
  name: 'parent3'
}];



let b = [{
  id: 1,
  nested: [{
    id: 10,
    nestedName: 'NestName1',
    count: 3
  }, {
    id: 2,
    nestedName: 'NestName2',
    count: 3
  }],
  name: 'parent1'
}, {
  id: 2,
  nested: [{
    id: 18,
    nestedName: 'NestName3',
    count: 3
  }, {
    id: 2,
    nestedName: 'NestName2',
    count: 3
  }, {
    id: 20,
    nestedName: 'NestName4',
    count: 3
  }],
  name: 'parent2'
}, {
  id: 3,
  nested: [{
    id: 5,
    nestedName: 'NestName5',
    count: 3
  }, {
    id: 30,
    nestedName: 'NestName6',
    count: 3
  }, {
    id: 20,
    nestedName: 'NestName4',
    count: 3
  }],
  name: 'parent3'
}];

//Подсчитать сумму поля count элементов вложенной коллекции nested из коллекции a, которые встречаються во вложенных коллекциях nested коллекции b. Совпадение элементов вложенных коллекций проверяем по равенству поля id. 

let col2 = _.chain(b)
  .map((parent) => parent.nested)
  .flatten().value();
  
let agg = _.chain(a)
  .map((parent) => {
    return {
      name: parent.name,
      aggregatedCount: _.reduce(parent.nested, (cumVal, nestObj) =>
     cumVal + (_.some(col2, (nestControlVal) => nestControlVal.id === nestObj.id) ? nestObj.count :...