How do you iterate over object properties?

The short answer

The main ways are Object.keys() (get keys), Object.values() (get values), Object.entries() (get key-value pairs), and for...in (loop over all enumerable properties including inherited ones). Object.keys/values/entries are preferred because they only return the object's own properties.

Object.keys

Returns an array of the object's own property names:

1const user = { name: 'John', age: 30, city: 'NYC' };
2
3Object.keys(user); // ["name", "age", "city"]
4
5Object.keys(user).forEach((key) => {
6  console.log(`${key}: ${user[key]}`);
7});

Object.values

Returns an array of the values:

1Object.values(user); // ["John", 30, "NYC"]

Object.entries

Returns an array of [key, value] pairs — the most useful for iteration:

1Object.entries(user); // [["name", "John"], ["age", 30], ["city", "NYC"]]
2
3for (const [key, value] of Object.entries(user)) {
4  console.log(`${key}: ${value}`);
5}

for...in

Loops over all enumerable properties, including inherited ones:

1for (const key in user) {
2  console.log(`${key}: ${user[key]}`);
3}

If you use for...in, always check hasOwnProperty to skip inherited properties:

1for (const key in user) {
2  if (user.hasOwnProperty(key)) {
3    console.log(`${key}: ${user[key]}`);
4  }
5}

Or just use Object.keys() / Object.entries() which do not include inherited properties.

Why interviewers ask this

This tests basic JavaScript knowledge. Interviewers want to see if you know the different methods and which ones include inherited properties.