const userProfile = {
  id: 1,
  name: {
    first: "John",
    last: "Doe"
  },
  age: 30,
  address: {
    street: "123 Main St",
    city: "Springfield",
    state: "IL",
    zip: "62704",
    country: "USA"
  },
  contacts: {
    email: "john.doe@example.com",
    phone: {
      home: "555-1234",
      work: "555-5678",
      mobile: "555-8765"
    }
  },
  preferences: {
    notifications: {
      email: true,
      sms: false,
      push: true
    },
    theme: "dark",
    language: "en-US"
  },
  friends: [
    {
      id: 2,
      name: "Jane Smith",
      email: "jane.smith@example.com"
    },
    {
      id: 3,
      name: "Mike Johnson",
      email: "mike.johnson@example.com"
    }
  ],
  active: true,
  createdAt: "2020-01-01T12:00:00Z"
};

const obj = {a:1, b:{c:{d:3,e:[4,5,6]}}}

const flattenObj = (obj, objKey="") => {
  let flat = {}
    
  for(const [key, value] of Object.entries(obj) ){
    if(typeof value === "object" && !Array.isArray(value)){
         flat = { ...flat ,...flattenObj(value, objKey?objKey+"."+key:key)}
    }else{
        flat[`${objKey?objKey+"."+key:key}`]=value
    }
  }
  return flat
}

console.log(flattenObj(userProfile))