Object Builder (arrayToObjectParser) : Provided an array of strings, the program returns an output object with key in correct hierarchy.

arrayToObjectParser.js

/* 
  Author: Shourya Ranka
  Objective : Provided an array of strings, the program returns an output object with key in correct hierarchy.
  input : ["root", "root.user", "root.user.a", "root.a"]
  output : {
      root: {
        a:{},
        user:{
          a:{}
        }
      }
  }
*/

const arrayToObjectParser = (arrayInput) => {
    if(!arrayInput || !arrayInput.length){
        return {};
    }
    let result = {};
    arrayInput.forEach(function(item, index){
        let itemsList = item.split(".");
        if(itemsList.length > 1){
           result[itemsList[0]] = result[itemsList[0]] || {};
           let pointer = result[itemsList[0]];
           for(let i=1; i<itemsList.length;i++){
              pointer[itemsList[i]] = {};
              pointer = pointer[itemsList[i]];
           }
        }
        else{
          result[item] = {};
        }
    });
    return result;
}

// Test 1 : Ideal case input
console.log(arrayToObjectParser(["root", "root.user", "root.user.a", "root.a"]));
// Test 2 : Input is empty
console.log(arrayToObjectParser([]));
// Test 3 : Input is invalid
console.log(arrayToObjectParser());
// Test 4 :  Parent item is not provided
console.log(arrayToObjectParser(["root.user", "root.user.a", "root.a"]));
// Test 5 : Only nested key is provided
console.log(arrayToObjectParser(["root.user.a"]));

Code Execution @ https://codepen.io/sranka2310/pen/VwWPqyL