Flatten a nested object into a flat object keyed by path strings

flatten.js

const flatten = (obj, flattened, prevStringKey) => {
  Object.entries(obj).forEach(([key, value]) => {
    const stringKey = prevStringKey ? `${prevStringKey}.${key}` : key;
    if (typeof value === 'object') {
      flattened = flatten(value, flattened, stringKey);
    } else {
      flattened[stringKey] = value;
    }
  });
  return flattened;
};

const path = { in: { big: { object: true, another: false } }, out: { final: { countdown: 100 } } };

console.log(flatten(path, {}, 'path'));