const _ = require('lodash');

const flattenObj = obj => _.reduce(
    obj,
    (accObj, value, key) => {
      if (_.isObject(value)) {
        _.forEach(
            flattenObj(value),
            ($value, $key) => {
              accObj[`${key}.${$key}`] = $value;
            },
        );
      } else {
        accObj[key] = value;
      }
      return accObj;
    },
    {},
);

console.assert(
    flattenObj({
      a: {
        b: 1,
        d: {
          l: 3,
          k: 4,
        },
      },
      c: 3,
    }) ===
    {'a.b': 1, 'a.d.l': 3, 'a.d.k': 4, c: 3},
);