Flatten a Nested Object in JavaScript

Photo by cottonbro

Flattening a JavaScript Nested Object is a very common question I came across in technical interviews.

Take the following input object as an example.

const input = {
  name: 'Mansi',
  age: 25,
  department: {
    name: 'Customer Experience',
    section: 'Technical',
    branch: {
       name: 'Bangalore',
       timezone: 'IST'
    }
  },
  company: {
   name: 'SAP',
   customers: ['Ford', 'Nestle']
  },
  skills: ['javascript', 'node.js', 'html']
}

The expected output can be as follows:

const output = {
   name: 'Mansi',
   age: 25,
   department_name: 'Customer Experience',
   department_section: 'Technical',
   department_branch_name: 'Bangalore',
   department_branch_timezone: 'IST’,
   company_name: 'SAP',
   company_customers: ['Ford', 'Nestle'],
   skills: ['javascript', 'node.js', 'html']
}

We’ll start by discussing naive solutions and then transition to the final solution that would be better in the aspects of Space & Time Complexity.

We’ll use two for .. in to write a solution as follows:

const flattenObject = (input) => {
 let result = {};
 for (const key in input) {
  if (!input.hasOwnProperty(key)) {
    continue;
  } 
  if (typeof input[key] === "object" &&!Array.isArray(input[key])) {
        var subFlatObject…