Map and Set in JavaScript
In JavaScript, we usually use objects and arrays for most things.
Objects for key value data, Arrays for lists
But sometimes these are not enough.
Like:
you want keys that are not just strings
you want to store unique values only
you want better control over data
That’s where Map and Set come into the picture.
In this blog I will share how I understand Map and Set and when I actually use them.
Problem with objects and arrays
Before jumping into Map and Set, let’s see the problem.
Objects problem
const obj = {}
obj[1] = "one"
obj["1"] = "string one"
console.log(obj)
Output:
{ '1': 'string one' }
Here:
number 1 and string "1" become same key
So object forces keys to be strings.
Arrays problem
const arr = [1, 2, 2, 3]
Here:
duplicates are allowed
Sometimes we don’t want that.
What is Map
Map is like an object but better.
It stores data in key value pairs, but:
keys can be anything (number, string, object, function)
Example:
const map = new Map()
map.set(1, "one")
map.set("1", "string one")
console.log(map)
Map(1) { 1 => 'one' }
Map(2) { 1 => 'one', '1' => 'string one' }
Here both keys are different.
So:
Map does not convert keys to string
How Map feels
Think like:
Map = advanced key value storage
Getting values from Map
map.get(1) // "one"
map.get("1") // "string one"
Map key value visual
What is Set
Set is used to store only unique values.
That’s its main job.
Example:
const set = new Set([1, 2, 2, 3])
console.log(set)
Output:
{1, 2, 3}
duplicates automatically removed
How Set feels
Set = unique collection
No duplicates allowed
Adding values in Set
const set = new Set()
set.add(1)
set.add(2)
set.add(2)
console.log(set)
Still:
{1, 2}
Set visual
Map vs Object
Both store key value, but:
Object:
keys become string
limited flexibility
Map:
keys can be anything
keeps original type
better for dynamic data
Simple line:
Object = basic
Map = flexible
Set vs Array
Array:
allows duplicates
used for ordered list
Set:
only unique values
removes duplicates automatically
Simple line:
Array = list
Set = unique list
When to use Map
Use Map when:
keys are not just strings
you need key value structure
keys can be objects or numbers
you want more control than object
Example use case:
const user1 = { name: "A" }
const user2 = { name: "B" }
const map = new Map()
map.set(user1, "data1")
map.set(user2, "data2")
When to use Set
Use Set when:
you want unique values
removing duplicates
checking if value already exists
Example:
const nums = [1, 2, 2, 3]
const unique = new Set(nums)
console.log([...unique])
Map and Set are not used everywhere, but when needed, they make things much cleaner.
Map solves key limitations of objects
Set solves duplicate problems of arrays
