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                // helpers
const isObject = (obj) => ({} === '[object Object]')
const isObjectCollection = obj => Array.isArray(obj) || isObject(obj)

// walkObj utility function
const walkObj = (source, fn, parentKey) => {
    for (const [key, prop] of Object.entries(source)) {
        source[key] = isObjectCollection(prop)
          ? walkObj(prop, fn, key)
          : fn(key, prop, parentKey) ?? prop
    return source

// test object
const body = {
    id: 1,
    structuredDataNodes: { name: 'don\'t touch' },
    nestedData: {
        name: 'xxx',
        someData: {
            start: 'xxx',
            nestedArray: [{parentFolderPath: 'xxx'}]

// an object collection of replacements
const bodyReplacements = {
    'body-text': 'Body text from API',
    'start': '1478059200000',
    'name': 'New event from API',
    'parentFolderPath': 'news/events/2020/10'

// will return undefined if bodyReplacements doesn't have that property
const bodyEdits = (key, prop, parentKey) => {
    // exclude 'structuredDataNodes' here rather than in walkObj
    return (parentKey !== 'structuredDataNodes') 
        ? bodyReplacements[key]
        : undefined

const newBody = walkObj(
    JSON.parse(JSON.stringify(body)), // clone object first