Custom pick with support for arrays using asterisk

custom-pick.js

function get(object, path) {
  const fields = Array.isArray(path) ? path : path.split('.')
  let cursor = object
  
  for (let i = 0; i < fields.length; i++) {
    if (fields[i] === '*') { 
      // TODO: validation of cursor being an array
      const newPath = fields.slice(i + 1)
      return cursor.map(item => get(item, newPath))
    } else {
      cursor = cursor[fields[i]]
    }
  }
  
  return cursor
}

function set(object, path, value) {
  const fields = Array.isArray(path) ? path : path.split('.')
  let cursor = object
  let prevCursor
  
  for (let i = 0; i < fields.length - 1; i++) {
    const field = fields[i]
    
    if (field === '*') {
      const newPath = fields.slice(i + 1)
      // TODO: validation of value being an array
      const newValue = value.map((val, index) => set(cursor[index] || {}, newPath, val))
      
      if (i > 0) {
        prevCursor[fields[i - 1]] = newValue
      } else {
        return newValue
      }
      
      return object
    } else {
      prevCursor = cursor
      cursor = cursor[field] = cursor[field] || {}
    }
  }
  
  const lastField = fields[fields.length - 1]
  if (Array.isArray(cursor)) {
    cursor.forEach(item => item[lastField] = value)
  } else {
    cursor[lastField] = value
  }
  
  return object
}

function pick(object, paths) {
  return paths.reduce((slice, path) => {
    const value = get(object, path)
    
    if (typeof value !== 'undefined') {
      set(slice, path, value)
    }

    return slice
  }, {})
}

const payload = {
  title: 'test',
  nested: { 
    field: 3, 
    another: 4 
  },
  children: [
    { 
      field: 1, 
      another: 2, 
      obj: { 
        title: 'test1', 
        nested: [
          { a: 1, c: 3 }, 
          { a: 3 }
        ] 
      } 
    }, 
    { 
      field: 2, 
      another: 3, 
      obj: { 
        title: 'test2', 
        nested: [
          { a: 2, b: 2 }
        ] 
      } 
    }
  ]
}

console.log(pick(payload, [
  'title',
  'children.*.field', // asterisk mean "retrieve subpath from every array item. So, here we take `field` of every item inside `payload.children` array
  'children.*.obj.nested.*.a'
 ]))