Array of Objects in JavaScript: Create, Loop, Find, Filter, Sort and Group

An array of objects is a list where each item is an object with named properties — for example a list of users, products or test results. It's the most common data shape in JavaScript, and in test automation you meet it constantly: API responses in Playwright or Postman, test data, and rows read from a table.

Create an Array of Objects

const results = [
  { id: 1, name: 'Login with valid user', status: 'passed', durationMs: 1200 },
  { id: 2, name: 'Login with wrong password', status: 'failed', durationMs: 900 },
  { id: 3, name: 'Add item to cart', status: 'passed', durationMs: 2100 },
  { id: 4, name: 'Checkout as guest', status: 'skipped', durationMs: 0 },
];

console.log(results[0].name);   // Login with valid user

Square brackets select the item (results[0]); a dot selects the property (.name).

All the Operations in One Example

Every common operation, run with Node.js 22:

// An array of objects: each object is one test result
const results = [
  { id: 1, name: 'Login with valid user', status: 'passed', durationMs: 1200 },
  { id: 2, name: 'Login with wrong password', status: 'failed', durationMs: 900 },
  { id: 3, name: 'Add item to cart', status: 'passed', durationMs: 2100 },
  { id: 4, name: 'Checkout as guest', status: 'skipped', durationMs: 0 },
];

// 1. Read a value
console.log('1.', results[0].name);

// 2. Loop over every object
for (const r of results) console.log('2.', r.id, r.status);

// 3. find: the first match (or undefined)
const firstFailed = results.find(r => r.status === 'failed');
console.log('3.', firstFailed.name);

// 4. filter: every match, as a new array
const passed = results.filter(r => r.status === 'passed');
console.log('4.', passed.length, 'passed');

// 5. map: turn each object into something else
console.log('5.', results.map(r => r.name));

// 6. some / every: yes-or-no questions about the whole array
console.log('6.', results.some(r => r.status === 'failed'), results.every(r => r.durationMs < 3000));

// 7. reduce: total time
const total = results.reduce((sum, r) => sum + r.durationMs, 0);
console.log('7.', total, 'ms in total');

// 8. sort by a property (copy first: sort() changes the original)
const slowest = [...results].sort((a, b) => b.durationMs - a.durationMs);
console.log('8.', slowest.map(r => `${r.id}:${r.durationMs}`).join(' '));

// 9. add, update and remove
results.push({ id: 5, name: 'Logout', status: 'passed', durationMs: 300 });
const cart = results.find(r => r.id === 3); cart.status = 'failed';
const withoutSkipped = results.filter(r => r.status !== 'skipped');
console.log('9.', results.length, 'results,', withoutSkipped.length, 'without skipped');

// 10. group by status
const byStatus = results.reduce((acc, r) => { (acc[r.status] ||= []).push(r.id); return acc; }, {});
console.log('10.', byStatus);

// 11. to and from JSON (API responses are usually arrays of objects)
const json = JSON.stringify(passed.map(({ id, name }) => ({ id, name })));
console.log('11.', json);
console.log('11.', JSON.parse(json)[1].name);

Output

1. Login with valid user
2. 1 passed
2. 2 failed
2. 3 passed
2. 4 skipped
3. Login with wrong password
4. 2 passed
5. [
  'Login with valid user',
  'Login with wrong password',
  'Add item to cart',
  'Checkout as guest'
]
6. true true
7. 4200 ms in total
8. 3:2100 1:1200 2:900 4:0
9. 5 results, 4 without skipped
10. { passed: [ 1, 5 ], failed: [ 2, 3 ], skipped: [ 4 ] }
11. [{"id":1,"name":"Login with valid user"},{"id":3,"name":"Add item to cart"}]
11. Add item to cart

Using It in Playwright API Tests

const response = await request.get('/api/users');
const users = await response.json();                 // an array of objects
expect(users.length).toBeGreaterThan(0);
expect(users.some(u => u.email === 'admin@example.com')).toBeTruthy();
const admin = users.find(u => u.role === 'admin');
expect(admin).toBeDefined();

Common Mistakes

const a = [{ id: 1 }, { id: 2 }];
console.log('includes:', a.includes({ id: 1 }));                  // compares references, not contents
console.log('some:', a.some(o => o.id === 1));
const copy = [...a]; copy[0].id = 99;                              // spread copies the array, not the objects
console.log('original after editing the copy:', a[0].id);
const deep = structuredClone(a); deep[1].id = 42;
console.log('original after editing a deep copy:', a[1].id);
console.log('sort numbers as text:', [10, 9, 100].sort());
console.log('find with no match:', [{ id: 1 }].find(o => o.id === 5));

Output

includes: false
some: true
original after editing the copy: 99
original after editing a deep copy: 2
sort numbers as text: [ 10, 100, 9 ]
find with no match: undefined

1. includes() with an object returns false — objects are compared by reference, not contents. Fix: some(o => o.id === 1).
2. Spread copies the array, not the objects — editing copy[0] changed the original. Fix: structuredClone(array) for a deep copy.
3. sort() without a compare function sorts as text (10, 100, 9). Fix: sort((a, b) => a - b) or compare a property.
4. sort() changes the original array. Fix: sort a copy: [...arr].sort(...) (or toSorted() in newer engines).
5. Using find()'s result without checking — it returns undefined when nothing matches, and .name then throws a TypeError.

Practice Exercises

1. From the results array, print the names of all failed tests.
2. Calculate the average duration of passed tests.
3. Group the results by status and print how many are in each group.

Key Takeaways

• find returns one item; filter and map return new arrays; reduce returns one value.
• Objects are compared by reference — use some/find with a property check.
• sort() sorts as text by default and changes the original array.

📚 Official documentation: MDN: Array · MDN: structuredClone()

Frequently Asked Questions

How do I find an object in an array by a property?

Use find: users.find(u => u.id === 3). It returns the first match or undefined.

How do I sort an array of objects by a property?

Pass a compare function: arr.sort((a, b) => a.age - b.age) for numbers, or (a, b) => a.name.localeCompare(b.name) for text. Sort a copy if you need the original order.

How do I check if an array of objects contains a value?

Use some: arr.some(o => o.email === "x@y.com"). includes() compares object references, so it returns false for a new object with the same contents.

How do I copy an array of objects?

[...arr] copies the array but shares the objects. Use structuredClone(arr) for a deep copy.

How do I convert an array of objects to JSON?

JSON.stringify(arr) turns it into a JSON string; JSON.parse(text) turns it back into an array of objects.