Iterating over objects in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the simplest way to get all keys from a JavaScript object?

Use Object.keys(object). It returns an array of all the object's own keys.

beginner

How do you loop over all key-value pairs in an object using for...in?

Use <code>for (const key in object) { const value = object[key]; }</code>. It loops over all enumerable keys, including inherited ones.

beginner

What method returns an array of an object's own enumerable property [key, value] pairs?

Use Object.entries(object). It returns an array of arrays, each with [key, value].

intermediate

Why should you use Object.hasOwn(object, key) inside a for...in loop?

To check if the key belongs directly to the object and not inherited from its prototype chain.

beginner

How can you iterate over an object’s keys and values using Object.entries and for...of?

Use <code>for (const [key, value] of Object.entries(object)) { /* use key and value */ }</code> for clean and direct access.

Which method returns an array of an object's keys?

A Object.keys()

B Object.values()

C Object.entries()

D for...in loop

Object.keys() returns an array of the object's own keys.

What does for...in loop iterate over?

A Only own enumerable keys

B Only values of the object

C Only symbols

D All enumerable keys including inherited ones

for...in loops over all enumerable keys, including inherited keys.

Which method gives you both keys and values as pairs in an array?

A Object.entries()

B Object.values()

C Object.keys()

D for...of loop

Object.entries() returns an array of [key, value] pairs.

Why use Object.hasOwn(object, key) inside a for...in loop?

A To convert object to array

B To get values

C To skip inherited keys

D To get keys

It ensures you only process keys that belong directly to the object.

Which loop works best with Object.entries() for clean key-value access?

A for...in

B for...of

C while

D do...while

for...of loops over arrays like the one returned by Object.entries().

Explain how to iterate over all keys and values of a JavaScript object safely.

Think about methods that return keys or entries and how to avoid inherited keys.

Describe the difference between for...in and for...of when iterating over objects.

Consider what each loop expects to work with and what they return.