JavaScript Sets — Interview Questions & Answers

Ques: What is a Set in JavaScript?

Ans: A Set is a collection of unique values, meaning it automatically removes duplicates. It can store any type of value — primitives or object references.

const mySet = new Set([1, 2, 2, 3]);
console.log(mySet); // Set(3) {1, 2, 3}

Ques: How is a Set different from an Array?

Feature	Set	Array
Duplicates	Not allowed	Allowed
Order	Insertion order	Indexed
Access	No index	Indexed by number
Performance	Faster for lookups	Slower for large datasets

Ques: How do you create a Set?

Ans: Using the Set() constructor:

const s1 = new Set();
const s2 = new Set([10, 20, 30]);

Ques: How do you add elements to a Set?

Ans: Use the .add() method:

const fruits = new Set();
fruits.add("apple");
fruits.add("banana");

Ques: How do you check if an element exists in a Set?

Ans: Use .has() method:

fruits.has("apple"); // true

Ques: How do you remove elements from a Set?

Ans: Use .delete() or .clear():

fruits.delete("apple");
fruits.clear(); // removes all

Ques: How do you find the size of a Set?

Ans: Use the .size property:

console.log(fruits.size);

Ques: How can you iterate over a Set?

Ans: You can use for...of, forEach, or convert it into an array.

for (let item of mySet) console.log(item);

Ques: How do you convert a Set to an Array?

Ans: Using the spread operator or Array.from():

const arr = [...mySet];

Ques: How do you remove duplicates from an Array using Set?

const nums = [1, 2, 2, 3];
const unique = [...new Set(nums)];
console.log(unique); // [1, 2, 3]

Ques: What are Set operations like Union, Intersection, and Difference?

Ans: You can perform these using spread and filter:

Union:

const union = new Set([...setA, ...setB]);

Intersection:

const intersection = new Set([...setA].filter(x => setB.has(x)));

Difference:

const difference = new Set([...setA].filter(x => !setB.has(x)));

Ques: Are Sets iterable?

Ans: Yes, You can loop through them with for...of or spread them.

Ques: Can a Set contain NaN or undefined?

Ans: Yes, Both are valid unique values in a Set.

new Set([NaN, NaN, undefined]).size; // 2

Ques: Can objects be stored in a Set?

Ans: Yes — but remember, each object has a unique reference.

const set = new Set();
set.add({a:1});
set.add({a:1});
console.log(set.size); // 2 (different references)

Ques: What is the WeakSet in JavaScript?

Ans: A WeakSet is similar to Set but stores only objects, not primitives.

It also holds weak references, meaning objects can be garbage-collected.

let obj = {name: "JS"};
const ws = new WeakSet();
ws.add(obj);
obj = null; // Object can be garbage collected

Ques: Difference between Set and WeakSet

Feature	Set	WeakSet
Data types	Any type	Only objects
Iteration	Iterable	Not iterable
Garbage collection	No	Yes (weak references)
Size property	Yes	No

Ques: Can WeakSets be iterated or converted to arrays?

Ans: No, because their contents are not enumerable — they exist for memory efficiency, not data iteration.

Ques: What are practical use cases of Sets?

• Removing duplicates
• Fast existence checks (has())
• Tracking unique user IDs or values
• Performing mathematical set operations