Merge data1.json and data2.json, and you get result.json

merge.js

const fs = require('fs/promises')
const util = require('util')

const process = (k1, v1, k2, v2) => {
  if (util.isDeepStrictEqual(v1, v2)) return v1
  if (typeof v1 === 'undefined' || v1 === null) return v2
  if (typeof v2 === 'undefined' || v2 === null) return v1
  let data = null
  if (Array.isArray(v1) !== Array.isArray(v2)) {
    if (Array.isArray(v1)) {
      throw new Error(`${k1} is array, but ${k2} is not an array`)
    } else {
      throw new Error(`${k2} is array, but ${k1} is not an array`)
    }
  }
  if ((typeof v1 === 'object') !== (typeof v2 === 'object')) {
    if (typeof v1 === 'object') {
      throw new Error(`${k1} is object, but ${k2} is not an object`)
    } else {
      throw new Error(`${k2} is object, but ${k1} is not an object`)
    }
  }
  if (Array.isArray(v1)) {
    data = v1
    data.push(...v2)
  } else if (typeof v1 === 'object') {
    data = {}
    const keys = Object.keys(v1)
    keys.push(...Object.keys(v2))
    for (const key of keys.filter((v, i, a) => a.indexOf(v) === i)) {
      data[key] = process(`${k1}.${key}`, v1[key], `${k2}.${key}`, v2[key])
    }
  } else {
    if (v1 !== v2) {
      throw new Error(`Conflict: ${k1} = ${JSON.stringify(v1)} and ${k2} = ${JSON.stringify(v2)}`)
    }
  }
  return data
}

!(async () => {
  const data1 = JSON.parse((await fs.readFile('data1.json')).toString('utf8'))
  const data2 = JSON.parse((await fs.readFile('data2.json')).toString('utf8'))
  const data = process('data1', data1, 'data2', data2)
  await fs.writeFile('result.json', JSON.stringify(data), { encoding: 'utf8' })
})()