Recursive Map function

Question

I'm having hard time figuring out how to do this recursive map function. I have an array that look like this.

var array = [
      {
        id: 1,
        label: 'Satisfied customers',
        children: [
          {
            id: 2,
            label: 'Good food',
            icon: 'restaurant_menu',
            children: [

              { id: 3, label: 'Quality ingredients'},
              { id: 4, label: 'Good recipe' }
            ]
          },
          {
            id: 5,
            label: 'Good service',
            icon: 'room_service',
            children: [
              { id: 6, label: 'Prompt attention' },
              { id: 7, label: 'Professional waiter' }
            ]
          },
          {
            id: 8,
            label: 'Pleasant surroundings',
            icon: 'photo',
            children: [
              {
                id: 9,
                label: 'Happy atmosphere (not tickable)',
                tickable: false,
              },
              {
                id: 10,
                label: 'Good table presentation (disabled node)',
                disabled: true,
              },
              {
                id: 11,
                label: 'Pleasing decor',
              }
            ]
          },
          {
            id: 12,
            label: 'Extra information (has no tick)',
            noTick: true,
            icon: 'photo'
          },
          {
            id: 13,
            label: 'Forced tick strategy (to "strict" in this case)',
            tickStrategy: 'strict',
            icon: 'school',
            children: [
              {
                id: 14,
                label: 'Happy atmosphere',
              },
              {
                id: 15,
                label: 'Good table presentation',
              },
              {
                id: 16,
                label: 'Very pleasing decor',
              }
            ]
          }
        ]
      }
    ];

This is the array looks like...

As you can see the children is recursive.

I need to put them into one array. My code doesn't work an has an error.

const result = [];   
const map = (e) => {

    result.push({
        id: e.id,
        label: e.label,
    })

    e.children.map(map)


};

array.map(map);

the error is on e.children.map(map).

I need to push them all in array variable but I don't know how to do this. TY

Answer 1

You need to check if current item has children element, and you can use forEach instead because map return new array and forEach just go throw each element.

const cb = (e) => {
    res.push({
        id: e.id,
        label: e.label,
    });
    e.children && e.children.forEach(cb);
}
array.forEach(cb);

Answer 2

A great way to learn about mutual recursion using vanilla JavaScript -

const transform1 = ({ id = 0, label = "", children = [] }) =>
  [ { id, label }, ...transformAll (children) ] // calls transformAll

const transformAll = (children = []) =>
  children .flatMap (c => transform1 (c)) // calls transform1

console.log(transformAll(array))

Output -

[
  {
    "id": 1,
    "label": "Satisfied customers"
  },
  {
    "id": 2,
    "label": "Good food"
  },
  {
    "id": 3,
    "label": "Quality ingredients"
  },
  {
    "id": 4,
    "label": "Good recipe"
  },
  {
    "id": 5,
    "label": "Good service"
  },
  {
    "id": 6,
    "label": "Prompt attention"
  },
  {
    "id": 7,
    "label": "Professional waiter"
  },
  {
    "id": 8,
    "label": "Pleasant surroundings"
  },
  {
    "id": 9,
    "label": "Happy atmosphere (not tickable)"
  },
  {
    "id": 10,
    "label": "Good table presentation (disabled node)"
  },
  {
    "id": 11,
    "label": "Pleasing decor"
  },
  {
    "id": 12,
    "label": "Extra information (has no tick)"
  },
  {
    "id": 13,
    "label": "Forced tick strategy (to \"strict\" in this case)"
  },
  {
    "id": 14,
    "label": "Happy atmosphere"
  },
  {
    "id": 15,
    "label": "Good table presentation"
  },
  {
    "id": 16,
    "label": "Very pleasing decor"
  }
]

Expand the snippet below the verify the results in your own browser -

var array = [
      {
        id: 1,
        label: 'Satisfied customers',
        children: [
          {
            id: 2,
            label: 'Good food',
            icon: 'restaurant_menu',
            children: [

              { id: 3, label: 'Quality ingredients'},
              { id: 4, label: 'Good recipe' }
            ]
          },
          {
            id: 5,
            label: 'Good service',
            icon: 'room_service',
            children: [
              { id: 6, label: 'Prompt attention' },
              { id: 7, label: 'Professional waiter' }
            ]
          },
          {
            id: 8,
            label: 'Pleasant surroundings',
            icon: 'photo',
            children: [
              {
                id: 9,
                label: 'Happy atmosphere (not tickable)',
                tickable: false,
              },
              {
                id: 10,
                label: 'Good table presentation (disabled node)',
                disabled: true,
              },
              {
                id: 11,
                label: 'Pleasing decor',
              }
            ]
          },
          {
            id: 12,
            label: 'Extra information (has no tick)',
            noTick: true,
            icon: 'photo'
          },
          {
            id: 13,
            label: 'Forced tick strategy (to "strict" in this case)',
            tickStrategy: 'strict',
            icon: 'school',
            children: [
              {
                id: 14,
                label: 'Happy atmosphere',
              },
              {
                id: 15,
                label: 'Good table presentation',
              },
              {
                id: 16,
                label: 'Very pleasing decor',
              }
            ]
          }
        ]
      }
    ];
    
const transform1 = ({ id = 0, label = "", children = [] }) =>
  [ { id, label }, ... transformAll (children) ]
  
const transformAll = (children = []) =>
  children .flatMap (c => transform1 (c))
  
console.log(transformAll(array))

---

A tour of Flat Earth

If you've never seen .flatMap before -

xs.flatMap(f) == xs.map(f).reduce((a,b) => a.concat(b), [])
xs.flatMap(f) == xs.reduce((a,b) => a.concat(f(b)), [])

It's best seen with a simple demo -

const twice = x =>
  [ x, x ]
  
console .log
  ( [ 'a', 'b', 'c' ] .flatMap (twice) // [ 'a', 'a', 'b', 'b', 'c', 'c' ]
  , [ 1, 2, 3, 4, 5 ] .flatMap (twice) // [ 1, 1, 2, 2, 3, 3, 4, 4, 5, 5 ]
  )

flatMap is useful for all kinds of things -

const tree =
  [ 0, [ 1 ], [ 2, [ 3 ], [ 4, [ 5 ] ] ] ]
  
const all = ([ value, ...children ]) =>
  [ value, ...children .flatMap (all) ]
  
console .log (all (tree))
// [ 0, 1, 2, 3, 4, 5 ]

really cool things -

const ranks =
  [ 'J', 'Q', 'K', 'A' ]
  
const suits =
  [ '♡', '♢', '♤', '♧' ]

const cards =
  ranks .flatMap (r =>
  suits .flatMap (s =>
  [ [ r, s ] ]
  ))

console .log (cards)
// [ ['J','♡'], ['J','♢'], ['J','♤'], ['J','♧']
// , ['Q','♡'], ['Q','♢'], ['Q','♤'], ['Q','♧']
// , ['K','♡'], ['K','♢'], ['K','♤'], ['K','♧']
// , ['A','♡'], ['A','♢'], ['A','♤'], ['A','♧']
// ]