map function for objects in Node.js & JavaScript


Understanding the Map Function

The map function is traditionally used with arrays to execute a specified function on each array element and collect the results in a new array. However, when it comes to objects, there isn’t a built-in map function in JavaScript. Objects, unlike arrays, are not ordered collections, and they have key-value pairs to consider. Let’s venture into creating a custom map function to manipulate their properties with ease.


Creating a Map Function for Objects

To establish a map functionality for objects, we define our custom function that iterates over each key-value pair, applies a callback function, and constructs a new object with the transformed pairs. Let’s see this in action:

function mapObject(obj, callback) {
  let result = {};
  for (let key in obj) {
    if (obj.hasOwnProperty(key)) {
      result[key] = callback(key, obj[key]);
  return result;

Code Examples

Example 1: Capitalizing Object Values

Suppose you have an object where you want to capitalize each value. You can utilize the mapObject function like so:

function capitalize(value) {
  return value.toUpperCase();

const originalObject = { a: 'hello', b: 'world' };
const capitalizedObject = mapObject(originalObject, (key, value) => capitalize(value));



  a: 'HELLO',
  b: 'WORLD'

Example 2: Squaring Numeric Properties

Another scenario could involve squaring numeric values of an object’s properties:

const numbers = { x: 2, y: 3, z: 4 };
const squaredNumbers = mapObject(numbers, (key, value) => value * value);



  x: 4,
  y: 9,
  z: 16

Example 3: Modifying Object Keys

Modifying keys might also be necessary. In the following example, we prefix each key with ‘new_’:

const prefixes = { id: 1, name: 'Alice' };
const prefixedKeys = mapObject(prefixes, (key, value) => ({[`new_${key}`]: value}));



  new_id: 1,
  new_name: 'Alice'

Conclusive Summary

To summarize, the custom map function for objects allows developers to maintain the familiar and functional pattern of the array map function while adapting it to the unique structure of JavaScript objects. By using this mapping technique, developers can seamlessly transform objects’ values and keys by applying various operations, increasing code readability, and encouraging functional programming practices.

References and Further Reading

  • Mozilla Developer Network. “”. MDN Web Docs.