Data #2.5 — Object Methods & Utilities

JavaScript objects come with a set of static methods that let you inspect, create, copy, freeze, and iterate over objects. This is a practical reference for every Object.* method you'll reach for in interviews and real code.

1. Object.keys(obj) — Get Own Enumerable Keys

Returns an array of an object's own enumerable string-keyed property names.

Interview use: Iterating over object properties, counting keys for deep comparison, filtering keys in deepOmit.

2. Object.values(obj) — Get Own Enumerable Values

Returns an array of values (same order as Object.keys).

3. Object.entries(obj) — Get [key, value] Pairs

Returns an array of [key, value] pairs. Useful for iterating with destructuring.

Interview use: classnames() conditionally adds keys when values are truthy; listFormat style transformations.

4. Object.fromEntries(entries) — Array → Object

The reverse of Object.entries. Converts an iterable of [key, value] pairs into an object.

5. Object.assign(target, ...sources) — Copy Properties

Copies own enumerable properties from source objects to a target. Returns the target.

Interview use: Merging default options, shallow cloning, overriding configs.

6. Object.create(proto, descriptors?) — Create with Custom Prototype

Creates a new object with the specified prototype and optional property descriptors.

Interview use: Creating dictionaries without inherited properties (Object.create(null)), setting up prototype chains explicitly.

7. Object.freeze(obj) & Object.seal(obj) — Immutability

• freeze: No additions, deletions, or modifications. writable becomes false, configurable becomes false.
• seal: No additions or deletions, but existing properties can be modified. configurable becomes false.

Interview use: Ensuring configuration objects aren't mutated accidentally.

8. Object.defineProperty(obj, key, descriptor) — Precise Control

Define or modify a property with fine-grained control over writability, enumerability, and configurability.

9. Object.prototype.hasOwnProperty(key) — Own Property Check

Checks if a property exists directly on the object, not on the prototype chain.

Interview use: Deep equality checks, filtering prototype properties during iteration.

10. Object.prototype.toString.call(value) — Reliable Type Check

Returns [object Type] — the most reliable way to determine a value's internal type.

Interview use: Type utilities (#5), distinguishing arrays from plain objects, cross-realm type checking.

Quick Reference

Related Articles

• Implement deepClone()
• Implement deepEqual()
• Data #3 — Map, Set, WeakMap, WeakSet