test reduce vs for

const deepMerge = (target, source) => {
  return Object.keys(source).reduce((result, key) => {
  	if (Object.prototype.hasOwnProperty.call(source, key)) {
      result[key] = typeof source[key] === 'object' && source[key] !== null
        ? deepMerge(result[key] ?? {}, source[key])
        : source[key]
    }
    return result
  }, { ...target })
}
const english = {
    'key1': 'Hello',
    'key2': {
        'key3': 'Welcome here',
        'key4': 'optional item',
        'key5': {
            'key6': 'another level of nesting'
        }
    }
}
const french = {
    'key1': 'Bonjour',
    'key2': {
        'key3': 'Bienvenue'
    }
}
deepMerge(english, french)

const deepMerge = (target, source) => {
  const result = {}
  for (const key in target) {
  	if (Object.prototype.hasOwnProperty.call(source, key)) {
      result[key] = target[key]
    }
  }
  for (const key in source) {
  	if (Object.prototype.hasOwnProperty.call(source, key)) {
      if (typeof source[key] === 'object' && source[key] !== null) {
        if (!result[key]) {
          result[key] = Array.isArray(source[key]) ? [] : {}
        }
        result[key] = deepMerge(result[key], source[key])
      } else {
        result[key] = source[key]
      }
    }
  }
  return result
}

const english = {
    'key1': 'Hello',
    'key2': {
        'key3': 'Welcome here',
        'key4': 'optional item',
        'key5': {
            'key6': 'another level of nesting'
        }
    }
}
const french = {
    'key1': 'Bonjour',
    'key2': {
        'key3': 'Bienvenue'
    }
}
deepMerge(english, french)