Flatten nested object

flatten.js

function flatten(obj, key_id = '') {
  let newObj = {};
  const keyPrefix = key_id + (key_id !== '' ? '.' : '');
  
  for(let key in obj) {
    if (typeof obj[key] === 'object') {
      newObj = Object.assign({}, newObj, flatten(obj[key], key));
    } else {
      newObj[keyPrefix + key] = obj[key];
    }
  }
  
  return newObj;
}

const start = {
  a: 'b',
  c: {
    d: 'e',
    f: 'g',
    h: {
      i: 'j',
      k: 'l',
      m: {
        n: 'o'
      }
    }
  },
  p: 'q'
};

console.log('flatten', flatten(start));