Flatten a nested object and sort by alphabetical order.

location.js

const obj = [
  {
    city: "Kampong Thom",
    suburbs: [
      {
        suburb: "Stueng Saen",
        suburbs: [
          "Achar Leak",
            "Damrei Choan Khla",
            "Kampong Krabau",
            "Kampong Roteh",
            "Kampong Thum",
            "Ou Kanthor",
            "Prey Ta Hu",
            "Srayov"
        ]
      },
      {
        suburb: "Baray",
        suburbs: [
          "Andoung Pou",
            "Bak Sna",
            "Ballangk",
        ]
      }
    ]
  },
  {
    city: "Banteay Meanchey",
    suburbs: [
       {
         suburb: "Malai",
         suburbs: [
            "Malai",
            "Ou Sampor",
            "Ou Sralau",
            "Ta Kong",
            "Tuol Pongro",
            "boeung Beng"
          ]
       }
    ]
  },
  {
     city: "Phnom Penh",
     suburbs: [
        { suburb: "7 Makara", suburbs: [] }
     ]
  }
];

const flattenLoc = [];

function flatten(data) {
    for (let city of data)  {
      const section = city.city.charAt(0);
      const newSec = _.toInteger(section) ? '#' : section;
      flattenLoc.push({ 
        section: transformPascalCase(newSec),
        label: city.city, 
        value: transformPascalCase(city.city),
      });
      for(let sub of city.suburbs) {
        const section = sub.suburb.charAt(0);
        const newSec = _.toInteger(section) ? '#' : section;
        flattenLoc.push({ 
          section: transformPascalCase(newSec),
          label: sub.suburb, 
          value: transformPascalCase(sub.suburb),
        });
        for (let khan of sub.suburbs) {
          const section = khan.charAt(0);
          const newSec = _.toInteger(section) ? '#' : section;
          flattenLoc.push({ 
            section: transformPascalCase(newSec),
            label: khan, 
            value: transformPascalCase(khan),
          });
        }
      }
    }
}
function transformPascalCase(str) {
  return str.replace(/\w\S*/g, function(txt){return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();});
}

flatten(obj);

const orderedLoc = _.orderBy(flattenLoc, ['label'], ['asc', 'desc']);
const sortByAlpha = _.groupBy(flattenLoc, 'section');
let sections = ['#', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 
               'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'X', 'Y', 'Z'];
// console.log(sortByAlpha);
const n = _.groupBy(orderedLoc, 'section')
console.log(n);
for (var i in n) {
  console.log(i);
}