JavaScript Arrays and Objects Practice

Practise transforming data with map, filter, reduce and sort, working with objects, destructuring, Map, Set and JSON. Every solution has been run with Node.js and shows its real output.

Intermediate7 problemsUpdated October 24, 2026

Review first: Arrays, Array Methods: map, filter, reduce, Objects, Destructuring and Spread, Map and Set, JSON

Several problems use this list of orders:

JavaScript

const orders = [
  { id: 1, customer: "Asha", amount: 499, status: "paid" },
  { id: 2, customer: "Ravi", amount: 1200, status: "pending" },
  { id: 3, customer: "Asha", amount: 250, status: "paid" },
  { id: 4, customer: "Meera", amount: 80, status: "cancelled" },
];

1.Total of paid orders

Beginner

Print the total amount of the orders whose status is "paid".

Show hint

filter the paid orders, then reduce to a sum starting at 0.

Show solution

JavaScript

const orders = [
  { id: 1, customer: "Asha", amount: 499, status: "paid" },
  { id: 2, customer: "Ravi", amount: 1200, status: "pending" },
  { id: 3, customer: "Asha", amount: 250, status: "paid" },
  { id: 4, customer: "Meera", amount: 80, status: "cancelled" },
];
const total = orders
  .filter((o) => o.status === "paid")
  .reduce((sum, o) => sum + o.amount, 0);
console.log(total);

Output

2.Sorted summary strings

Beginner

Print an array of strings like "#2 Ravi ₹1200" for every order that is not cancelled, sorted from the largest amount to the smallest.

Show hint

filter, then toSorted((a, b) => b.amount - a.amount), then map to strings.

Show solution

JavaScript

const orders = [
  { id: 1, customer: "Asha", amount: 499, status: "paid" },
  { id: 2, customer: "Ravi", amount: 1200, status: "pending" },
  { id: 3, customer: "Asha", amount: 250, status: "paid" },
  { id: 4, customer: "Meera", amount: 80, status: "cancelled" },
];
const summary = orders
  .filter((o) => o.status !== "cancelled")
  .toSorted((a, b) => b.amount - a.amount)
  .map((o) => `#${o.id} ${o.customer} ₹${o.amount}`);
console.log(summary);

Output

[ '#2 Ravi ₹1200', '#1 Asha ₹499', '#3 Asha ₹250' ]

3.Totals per customer

Intermediate

Build an object that maps each customer's name to the total of all their orders (any status), and print it.

Show hint

reduce into an object: totals[name] = (totals[name] ?? 0) + amount.

Show solution

JavaScript

const orders = [
  { id: 1, customer: "Asha", amount: 499, status: "paid" },
  { id: 2, customer: "Ravi", amount: 1200, status: "pending" },
  { id: 3, customer: "Asha", amount: 250, status: "paid" },
  { id: 4, customer: "Meera", amount: 80, status: "cancelled" },
];
const totals = orders.reduce((acc, { customer, amount }) => {
  acc[customer] = (acc[customer] ?? 0) + amount;
  return acc;
}, {});
console.log(totals);

Output

{ Asha: 749, Ravi: 1200, Meera: 80 }

4.Unique customers

Beginner

Print the unique customer names in the order they first appear, and how many there are.

Show hint

new Set(...) removes duplicates; spread it back into an array.

Show solution

JavaScript

const orders = [
  { id: 1, customer: "Asha", amount: 499, status: "paid" },
  { id: 2, customer: "Ravi", amount: 1200, status: "pending" },
  { id: 3, customer: "Asha", amount: 250, status: "paid" },
  { id: 4, customer: "Meera", amount: 80, status: "cancelled" },
];
const customers = [...new Set(orders.map((o) => o.customer))];
console.log(customers, customers.length);

Output

[ 'Asha', 'Ravi', 'Meera' ] 3

5.Update an object without changing it
Intermediate
Given const user = { name: "Asha", address: { city: "Pune", pin: "411001" } }, create moved, a copy where the city is "Delhi", without changing user. Print both cities.
Show hint
Spread the object and the nested object: { ...user, address: { ...user.address, city: "Delhi" } }.
Show solution
JavaScript
const user = { name: "Asha", address: { city: "Pune", pin: "411001" } }; const moved = { ...user, address: { ...user.address, city: "Delhi" } }; console.log(user.address.city, moved.address.city);
Output
Pune Delhi

6.Most common words

Intermediate

Count how often each word appears in "the cat and the hat and the bat" using a Map, then print the two most common words with their counts.

Show hint

Count with map.set(word, (map.get(word) ?? 0) + 1), then sort [...map] by count.

Show solution

JavaScript

const text = "the cat and the hat and the bat";
const counts = new Map();
for (const word of text.split(" ")) {
  counts.set(word, (counts.get(word) ?? 0) + 1);
}
const top = [...counts].sort((a, b) => b[1] - a[1]).slice(0, 2);
console.log(top);

Output

[ [ 'the', 3 ], [ 'and', 2 ] ]

7.JSON round trip

Beginner

Convert { course: "JavaScript", lessons: 56, tags: ["web", "frontend"] } to a JSON string and print it, then parse it back and print the second tag.

Show hint

JSON.stringify and JSON.parse.

Show solution

JavaScript

const data = { course: "JavaScript", lessons: 56, tags: ["web", "frontend"] };
const json = JSON.stringify(data);
console.log(json);
console.log(JSON.parse(json).tags[1]);

Output

{"course":"JavaScript","lessons":56,"tags":["web","frontend"]}
frontend

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