JavaScript filter with recursive relationships

Question

My use case is something like this.

I have an array of objects called locations.
It contains information about Provinces, Districts and cities.
City belongs to a District, District belongs to a Province.
If parent_region_id null means it is a province.
If parent_region_id is a province's id means it is a district.
Otherwise it's a city.

My goal is to filter out all the cities from the array. I did a odd way and it works though.

But I would like to know what is most optimal way to do this? This is my working code.

const locations = [
{id:1,name:"Western province",parent_region_id:null},
{id:2,name:"Southern province",parent_region_id:null},
{id:3,name:"Central province",parent_region_id:null},
{id:4,name:"Colombo district",parent_region_id:1},
{id:5,name:"Galle district",parent_region_id:2},
{id:6,name:"Kandy district",parent_region_id:3},
{id:7,name:"Maharagama",parent_region_id:4},
{id:8,name:"Nugegoda",parent_region_id:4},
{id:9,name:"Peradeniya",parent_region_id:6},
]

//get the province ids first

const provinces = []

locations.forEach(e=> {
if(!e.parent_region_id){
  provinces.push(e.id)
}
})

// get all the districts then

const districts = []

locations.forEach(e=>{
  if(provinces.includes(e.parent_region_id)){
  districts.push(e.id) 
  }
})


//get cities

const cities = [];


locations.forEach(e=> 
{
  if(!districts.includes(e.id) && !provinces.includes(e.id)){
    console.log(e.name," is a city")
  }
})

kemicofa · Accepted Answer · 2019-01-21 11:12:57Z

Use Array#filter and Array#find.

const locations=[{id:1,name:"Western province",parent_region_id:null},{id:2,name:"Southern province",parent_region_id:null},{id:3,name:"Central province",parent_region_id:null},{id:4,name:"Colombo district",parent_region_id:1},{id:5,name:"Galle district",parent_region_id:2},{id:6,name:"Kandy district",parent_region_id:3},{id:7,name:"Maharagama",parent_region_id:4},{id:8,name:"Nugegoda",parent_region_id:4},{id:9,name:"Peradeniya",parent_region_id:6},]

const res = locations.filter(({id, parent_region_id},i,a)=>{
  if(!parent_region_id) return false;
  const loc = a.find(l=>l.id === parent_region_id);
  return !loc.parent_region_id ? false : true;
});

console.log(res);

Solution by removing the provinces first:

const locations=[{id:1,name:"Western province",parent_region_id:null},{id:2,name:"Southern province",parent_region_id:null},{id:3,name:"Central province",parent_region_id:null},{id:4,name:"Colombo district",parent_region_id:1},{id:5,name:"Galle district",parent_region_id:2},{id:6,name:"Kandy district",parent_region_id:3},{id:7,name:"Maharagama",parent_region_id:4},{id:8,name:"Nugegoda",parent_region_id:4},{id:9,name:"Peradeniya",parent_region_id:6},]

const res = locations
.filter(({parent_region_id})=>parent_region_id!==null)
.filter(({parent_region_id},i,a)=>{
  return a.findIndex(l=>l.id === parent_region_id) > -1;
});

console.log(res);

Jack Bashford · Accepted Answer · 2019-01-21 11:11:55Z

If you want to make the code cleaner and more compact, try using filter() like so:

 

const locations = [
{id:1,name:"Western province",parent_region_id:null},
{id:2,name:"Southern province",parent_region_id:null},
{id:3,name:"Central province",parent_region_id:null},
{id:4,name:"Colombo district",parent_region_id:1},
{id:5,name:"Galle district",parent_region_id:2},
{id:6,name:"Kandy district",parent_region_id:3},
{id:7,name:"Maharagama",parent_region_id:4},
{id:8,name:"Nugegoda",parent_region_id:4},
{id:9,name:"Peradeniya",parent_region_id:6},
];

const provinces = locations.filter(e => !e.parent_region_id);

const districts = locations.filter(e => provinces.some(p => p.id == e.parent_region_id));

const cities = locations.filter(e => !provinces.includes(e) && !districts.includes(e));

console.log(provinces);
console.log(districts);
console.log(cities);

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