flatObject.js

// Using recursion to flat object.

function magic(obj, name) {
  let myObj = {};
  for (let key in obj) {
    if (typeof obj[key] !== 'object') {
      myObj[name + '_' + key] = obj[key];
    } else {
      const magicValue = magic(obj[key], name + '_' + key);
      // for (let i in magicValue) {
      //   myObj[name + '_' + i] = magicValue[i];
      // }
      myObj = {
         ...myObj,
        ...magicValue
      }
    }
  }
  return myObj;
}


let user = {
  name: {
    firstName: "Jack",
    lastName: "Reacher"
  },
  age: 25,
  address: {
    present: {
      houseNo: '110',
      locality: 'West Road',
      city: 'New York'

    },
    permanent: {
      houseNo: '7',
      locality: 'South Side',
      city: 'Riverdale'
    }
  },
  phoneNumber: 34500929
};

magic(user, 'user');