Flatten object keys

flattenObject.js

const flattenObject = obj => {
  if (typeof obj !== 'object') {
    return obj;
  }

  function flattenKeys(value, key, storedKey) {
    const newKey = storedKey ? `${storedKey}.${key}` : key;

    if (typeof value === 'object') {
      const firstKey = Object.keys(value)[0];
      return flattenKeys(value[firstKey], firstKey, newKey);
    }

    return { [newKey]: value };
  }

  return Object.keys(obj).reduce((memo, key) => {
    return { ...memo, ...flattenKeys(obj[key], key) };
  }, {});
};

describe('flattenObject', () => {
  it('returns value if not an object', () => {
    expect(flattenObject('string')).toEqual('string');
  });

  it('combines keys of nested objects', () => {
    const res = flattenObject({
      a: {
        b: {
          c: 1
        }
      },
      d: 2
    });
    expect(res).toEqual({ 'a.b.c': 1, d: 2 });
  });
});