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Map vs Object

map-vs-objectcollectionsmapkeysiteration

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Full lesson · 8 learning steps Open any exercise for focused practice

Step 1 of 8 · intro card

Learn Map vs Object

Objects & Maps

A Map stores key/value pairs with map.set, map.get, map.has, and map.size. Prefer a Map over a plain object when keys are not strings, when insertion order matters, or when you add and delete entries frequently.

A Map is a dedicated key/value collection: set(key, val) writes, get(key) reads, has(key) tests membership, and size reports the count directly.

Unlike a plain object, a Map accepts any value as a key — objects, functions, numbers — and never collides with prototype keys like __proto__ or toString.

Maps preserve insertion order, expose size in O(1), and are optimized for frequent additions and deletions, so reach for a Map when the object you would otherwise use is really a lookup table.

set / get / has / size are the core Map operations.

Map keys can be any value, not just strings.

Maps keep insertion order and report size directly.

const counts = new Map()
counts.set("ada", 1)
counts.set("ada", counts.get("ada") + 1)
const seen = counts.has("ada")
const total = counts.size
const objKey = new Map()
objKey.set({ id: 1 }, "first")

Read the implementation

Connect each part of the code to the concept before you type.

1. Create a Map

new Map() builds an empty Map that keeps keys in insertion order.

const counts = new Map()

2. set a value

set(key, value) stores or overwrites the value for a key and returns the Map.

counts.set("ada", 1)

3. get a value

get(key) returns the stored value, or undefined when the key is absent.

counts.get("ada")

4. has check

has(key) returns a boolean, the safe way to test membership.

counts.has("ada")

5. size property

size is the entry count, unlike a plain object which needs Object.keys(obj).length.

counts.size

6. Object as key

A Map accepts any value as a key, including objects — a plain object cannot.

objKey.set({ id: 1 }, "first")

A complete learning loop

Understand

Read the concept map and follow the code breakdown.

Reconstruct

Type the implementation, repair mistakes and work through every recall exercise.

Verify

Use the final check without hints; revisit weak concepts in short practice sessions.