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?

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

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