flatten-object.js

const isPrimitive = (x) => {
  const t = typeof x;

  return (
    t === 'string' ||
    t === 'number' ||
    t === 'boolean' ||
    t === 'undefined' ||
    t === null
  );
};

const buildPath = (path, key, value) => {
  const newPath = [...path, key];

  return isPrimitive(value)
    ? { [newPath.join('.')]: value }
    : Object.entries(value).reduce(
        (acc, [k, v]) => Object.assign(acc, buildPath(newPath, k, v)),
        {}
      );
};

const flatten = (obj) =>
  Object.entries(obj).reduce(
    (acc, [key, value]) => Object.assign(acc, buildPath([], key, value)),
    {}
  );

console.log(flatten({ foo: { bar: 1 } }));
//=> { 'foo.bar': 1 }

console.log(flatten({ foo: { bar: 1, baz: 2 } }));
//=> { 'foo.bar': 1, 'foo.baz': 2 }

console.log(flatten({ foo: { bar: 1, baz: ['a', 'b'] } }));
//=> { 'foo.bar': 1, 'foo.baz.0': 'a', 'foo.baz.1': 'b' }

console.log(flatten({ a: [{ foo: 1 }, { bar: 2 }] }));
//=> { 'a.0.foo': 1, 'a.1.bar': 2 }