Flatten a JSON object in TypeScript / JavaScript

flatten.ts

import _ from 'lodash';

function flattenObject(obj: any): any {
  function _flattenPairs(obj: any, prefix: string): [string, any][] {
    // console.log(obj, prefix);
    if (!_.isObject(obj)) {
      return [prefix, obj];
    }

    return _.toPairs(obj).reduce((final: [string, any][], nPair: [string, any]) => {
      const flattened = _flattenPairs(nPair[1], `${prefix}.${nPair[0]}`);
      if (flattened.length === 2 && !_.isObject(flattened[0]) && !_.isObject(flattened[1])) {
        return final.concat([flattened as [string, any]]);
      } else {
        return final.concat(flattened);
      }
    }, []);
  }

  if (!_.isObject(obj)) {
    return JSON.stringify(obj);
  }

  const pairs: [string, any][] = _.toPairs(obj).reduce((final: [string, any][], pair: [string, any]) => {
    const flattened = _flattenPairs(pair[1], pair[0]);
    if (flattened.length === 2 && !_.isObject(flattened[0]) && !_.isObject(flattened[1])) {
      return final.concat([flattened as [string, any]]);
    } else {
      return final.concat(flattened);
    }
  }, []);

  return pairs.reduce((acc: any, pair: [string, any]) => {
    acc[pair[0]] = pair[1];
    return acc;
  }, {});
}

console.log(JSON.stringify(flattenObject({
  a: {
    b: {
      c: 'd'
    },
    e: 'f'
  },
  g: 'h'
}), null, 2));

/*
prints:
{
  "a.b.c": "d",
  "a.e": "f",
  "g": "h"
}
*/