Iterate through object and flatten it.

recurseUnderscore.js

//Iterate through user object and flatter in one object.
//append _ on every level
var user = {
  name: "Shad",
  address: {
    personal: {
      city: "Varanasi",
      area: "Chauhatta",
    },
    office: {
      city: "Bangalore",
      area: {
        landmark: "Mantri Alpyne",
      }
    }
  }
}
var result = {};

function magic(user, parent){
  for (let key in user){
    if(typeof user[key] === 'object'){
      magic(user[key], parent + "_" + key);
    } else {
      result[parent + "_" + key] = user[key];
    }
  }
}
magic(user, 'user')
console.log(result);