JavaScript Set Object

**Set Object

Set

A Set stores unique values only (no duplicates) and maintains insertion order.
Syntax: const st = new Set();

Set Use Cases

Use when you need unique values (e.g., unique IDs, tags, categories).
Great for removing duplicates from arrays.
Fast search, add, delete operations compared to arrays.

Set Methods

.add()

Adds a value to the Set.
Syntax: st.add(value)

.has()

Checks whether a value exists.
Syntax: st.has(value)

.size

Returns number of unique items.
Syntax: st.size

.delete()

Removes a specific value.
Syntax: st.delete(value)

Interesting Fact Why Objects Behave Uniquely in Set

**1. Objects are stored by reference, not by value**

When you put objects inside a Set, JavaScript checks memory address, not values.

2. Your first object

let employee1 = { id: 1, name: "Arun" };

This object has its own memory address.
Example: 0x001 (just for explanation)

3. The array of employees

const employeeDetails = [ employee1, { id: 2, name: "Prasanna" } ];

Then you convert it into a Set:

const employeeDetailsSet = new Set(employeeDetails);

So the Set contains:

Object at address 0x001 → { id: 1, name: "Arun" }
Object at another address → { id: 2, name: "Prasanna" }

❌ Delete attempt 1

employeeDetailsSet.delete({ id: 1, name: "Arun" });

This looks the same but is a new object in memory, like:

{ id: 1, name: "Arun" } → address 0x999

Since 0x999 ≠ 0x001, Set will NOT delete it.

✅ Delete attempt 2

employeeDetailsSet.delete(employee1);

Here you pass the exact same object reference (employee1), same memory address 0x001.

So this time deletion works ✔️

🎯 Final Output Explanation

Before delete:
Set contains 2 objects.

After delete attempt 1:
Nothing removed (different reference).

After delete attempt 2:
employee1 is removed.

🧠 Key Takeaway for Interviews

In JavaScript, Sets treat objects based on reference, not value.
Two objects with identical values are NOT equal unless they refer to the same memory address.

.clear()

Removes all values.
Syntax: st.clear()

return value is undefined

.entries()

Returns [value, value] pairs (for compatibility with Map).
Syntax: st.entries()

.forEach()

Loops through values.
Syntax: st.forEach(val => { ... })

**WeakSet

A WeakSet stores only objects and allows automatic garbage collection.
Syntax: const ws = new WeakSet();

**WeakSet Use Cases

Track objects without preventing garbage collection
Store temporary state, e.g., "which objects are processed?"
Ideal for private object tracking in classes or modules

Comparison: Map vs Set vs WeakMap vs WeakSet

Feature	Map	Set	WeakMap	WeakSet
Stores	Key–value pairs	Unique values	Key–value pairs	Unique object values
Key Type	Any type (string, number, object)	Any value	Objects only	Objects only
Value Type	Any	Any	Any	Objects only
Duplicates Allowed?	No duplicate keys	No duplicate values	No duplicate keys	No duplicate values
Garbage Collection	❌ No	❌ No	✅ Yes (auto removes unused object keys)	✅ Yes (auto removes unused objects)
Iterable?	✅ Yes	✅ Yes	❌ No	❌ No
Has size property?	✅ `map.size`	✅ `set.size`	❌ No	❌ No
Methods	set, get, has, delete, clear, entries, forEach	add, has, delete, clear, entries, forEach	set, get, has, delete	add, has, delete
Order preserved?	Yes	Yes	No (not iterable)	No (not iterable)
Primary Use Case	Fast lookup table	Unique collection of values	Private data for objects	Track objects without memory leaks

1-Line Interview Descriptions

**Map

“Ordered key–value store where keys can be any type; good for fast lookups.”

**Set

“Stores unique values with fast membership checks; prevents duplicates.”

**WeakMap

“Map that stores only object keys with automatic garbage collection; not iterable.”

**WeakSet

“Set that stores only objects; auto garbage-collection; useful for tracking objects.”

Reference Code

Time & Space Complexity: Learn More

Source Link : https://github.com/er-arunkumarselvam/ES6--JavaScript/blob/main/assets/js/setObject.js