gistfile1.js

const deepEql = require('deep-eql')
const input = require('./input.json')
const output = require('./output.json')

const groupByDate = (messages) => messages.reduce(
    (carry, message) => {
	const copy = Array.from(carry)
	const tail = copy.pop();
	const groupDate = new Date(message.created_at).getDate();

	return tail && tail[0].groupDate === groupDate
	    ? copy.concat([tail.concat({ groupDate, ...message })])
	    : carry.concat([[{ groupDate, ...message }]])
    },
    []
);

const groupByUser = (messages) => messages.reduce(
    (carry, message) => {
	const copy = Array.from(carry)
	const tail = copy.pop()
	const { groupDate, ...rest } = message

	if (!tail || tail.user.id !== rest.user.id) {
	    return carry.concat({ ...rest, children: [rest] })
	}

	return copy.concat({ ...tail, children: tail.children.concat(rest) })
    },
    []
)

const combineMessages = (messages) => {
    const byDate = groupByDate(messages)

    return byDate.map(items => ({
	date: items[0].created_at,
	messages: groupByUser(items)
    }))
}

const result = _combineMessages(input)

if (!deepEql(output, result)) {
    console.log(JSON.stringify(result))
} else {
    console.log('ok!')
}