recursive function for nested array objects returns undefined

Question

I'm trying to find/ extract a specific object in a nested hierarchy. when the entity is found I console.log it and then return it. the log is the correct object but the function still returns undefined.

any solutions? I've tried with a map, reduce, and a classic for loop and still gets undefined back.

const hierarchy = {
  id: '1',
  title: 'root',
  children: [
    {
      id: '2',
      title: 'second ab',
      children: [
        {
          id:'999',
          title: 'nested child AB',
           children: [
          {
            id:'88888',
            title: 'very nested child AB'
          }
        ],
        }
      ],
    },
    {
      id: '3',
      title: 'third ab',
      children: [
        {
          id:'444',
          title: 'nested child AB',
           children: [
          {
            id:'5555',
            title: 'very nested child AB'
          }
        ],
        }
      ],
    }
  ]
}

const findEntityToAddInHierarchy = (entityArr, entityId) => {
     return entityArr.forEach(entity => {
       if(entity.id === entityId) {
         console.log('found and returning', entity)
         return entity;
       } else {
           return entity.children && findEntityToAddInHierarchy(entity.children, entityId)
       }
     })
  }


const result = findEntityToAddInHierarchy([hierarchy], '5555');

console.log('this returns undefined', result)

Hi! You can specify the language with a tag and in the body of your question; you can also add colour syntaxing by specifying the language in the code block, for instance writing ```javascript insert.code.here() ``` — Stef
– Stef, Commented Oct 8, 2020 at 16:56

Scott Sauyet · Accepted Answer · 2020-10-09 15:46:39Z

As imvrain2 already explained, there is an issue in your code with forEach. That answer is one way to solve this.

But I believe we can write this more simply. Here is a version that returns all matches in an array. If there are multiple values with the same id, they will all appear. If there are none, it will be an empty array. If you are using it with the certainty that three will always be exactly one, you can choose just to extract the first result from the list (although I suppose that might be wasteful.)

const matchById = (target) => (obj) => [
  ... (obj.id === target ? [obj] : []), 
  ... (obj .children || []) .flatMap (matchById (target))
]

const hierarchy = {id: '1', title: 'root', children: [{id: '2', title: 'second ab', children: [{id:'999', title: 'nested child AB', children: [{id:'88888', title: 'very nested child AB'}]}]}, {id: '3', title: 'third ab', children: [{id:'444', title: 'nested child AB', children: [{id:'5555', title: 'very nested child AB'}]}]}]}

console .log (matchById ('5555') (hierarchy))

We return an array that starts with the current object (if it has the right id) and then simply recurs on each of its children, flattening into a single array.

With only a little more work, however, we can make this more generic, where we test each element with a predicate to decide if we want to accept it, and then use that to build our custom function:

const getMatches = (pred) => (obj) => [
  ... (pred (obj) ? [obj] : []), 
  ... (obj .children || []) .flatMap (getMatches (pred))
]

const matchById = (target) =>
  getMatches (({id}) => id === target)

const hierarchy = {id: '1', title: 'root', children: [{id: '2', title: 'second ab', children: [{id:'999', title: 'nested child AB', children: [{id:'88888', title: 'very nested child AB'}]}]}, {id: '3', title: 'third ab', children: [{id:'444', title: 'nested child AB', children: [{id:'5555', title: 'very nested child AB'}]}]}]}

console .log (matchById ('5555') (hierarchy))

And now we can find all matches for any predicate we can define. This is the version I would use.

flatMap is a nice choice for finding all matches. nice work

Scott Sauyet · Accepted Answer · 2020-10-10 18:43:28Z

I think generators are a particularly good fit for searching algorithms. We can start by writing a generic preorder traversal which produces a flattened stream of objects. Then writing searchById is a simple for..of construct -

function* preorder (t = {})
{ yield t
  if (t.children)
    for (const c of t.children)
      yield* preorder(c)
}

function searchById (t = {}, q = null)
{ for (const v of preorder(t))
    if (v.id == q)
      return v
  return false
}

const input =
  {id: '1', title: 'root', children: [{id: '2', title: 'second ab', children: [{id:'999', title: 'nested child AB', children: [{id:'88888', title: 'very nested child AB'}]}]}, {id: '3', title: 'third ab', children: [{id:'444', title: 'nested child AB', children: [{id:'5555', title: 'very nested child AB'}]}]}]}

const result = 
  searchById(input, 5555)
  
console.log(result)

{
  "id": "5555",
  "title": "very nested child AB"
}

If no result is found, false is returned -

const result = 
  searchById(input, "Z")
  
console.log(result)

false

imvain2 · Accepted Answer · 2020-10-08 18:10:56Z

forEach doesn't return anything, so one solution is to set a variable instead. Then return the variable instead.

const hierarchy = {
  id: '1',
  title: 'root',
  children: [
    {
      id: '2',
      title: 'second ab',
      children: [
        {
          id:'999',
          title: 'nested child AB',
           children: [
          {
            id:'88888',
            title: 'very nested child AB'
          }
        ],
        }
      ],
    },
    {
      id: '3',
      title: 'third ab',
      children: [
        {
          id:'444',
          title: 'nested child AB',
           children: [
          {
            id:'5555',
            title: 'very nested child AB'
          }
        ],
        }
      ],
    }
  ]
}

const findEntityToAddInHierarchy = (entityArr, entityId) => {
     _r = {};
     entityArr.forEach(entity => {
       if(entity.id === entityId) {
         console.log('found and returning', entity)
         _r = entity;
       } else {
           _r = entity.children && findEntityToAddInHierarchy(entity.children, entityId)
       }
     })
     return _r
  }


const result = findEntityToAddInHierarchy([hierarchy], '5555');

console.log('this returns undefined', result)

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