Imagine you’re running a party RSVP list. People keep texting you their names—some twice, some even more! You don’t want to manually weed out duplicates. You just want a clean list of unique attendees.

Enter JavaScript’s Set, your automated guest bouncer! 👮‍♂️

A Set is a collection of values where each value must be unique. That means no duplicates allowed!

Let’s see how this works in practice.

🧪 Creating a Set

You create a new set like this:

let mySet = new Set();

Or you can initialize it with values:

let colors = new Set(["red", "blue", "green"]);

Duplicate values? No problem—they’ll be ignored:

let dupSet = new Set(["a", "b", "a", "c"]);
console.log(dupSet); // Set(3) { "a", "b", "c" }

🎯 Core Features of a Set

A JavaScript Set has the following features:

  • Uniqueness: No two values can be the same.
  • Order: The values are kept in the order of insertion.
  • Value Types: Any type of value can be stored—numbers, strings, objects, even other sets!

🔧 Common Set Methods

Let’s explore the tools in your Set toolbox.

✅ add(value)

Adds a value to the Set:

colors.add("yellow");

Even if you try adding “yellow” again, it won’t be duplicated.

❌ delete(value)

Removes the value if it exists:

colors.delete("blue");

❓ has(value)

Checks if a value exists:

console.log(colors.has("green")); // true

🧹 clear()

Removes all elements:

colors.clear();

📏 size

Gives the number of items in the Set:

console.log(colors.size); // 0

🔁 Iterating through a Set

You can use for...of:

for (let color of colors) {
  console.log(color);
}

Or go full pro with forEach:

colors.forEach(function(value) {
  console.log(value);
});

💡 Use Cases for Sets

  1. Removing duplicates from an array:
let nums = [1, 2, 3, 2, 4, 1];
let uniqueNums = [...new Set(nums)];
console.log(uniqueNums); // [1, 2, 3, 4]
  1. Checking existence without searching a whole array:
let users = new Set(["alice", "bob"]);
console.log(users.has("alice")); // true
  1. Storing flags or tags where order doesn’t matter but uniqueness does.

🧬 Set vs. Array: When Should You Use Set?

Feature Array Set
Allows duplicates ✅ Yes ❌ No
Easy to search (.includes()) ✅ Yes ✅ Yes (with .has())
Iteration ✅ Yes ✅ Yes
Order preserved ✅ Yes ✅ Yes (insertion order)
Performance (for large sets) Slower for lookups Faster for lookups

If your main goal is to store unique values and you don’t care about indices—Set is the clear winner.

📌 Summary

JavaScript’s Set is your go-to structure for handling unique collections. It’s lightweight, elegant, and makes your code cleaner—especially when dealing with filtering duplicates or storing non-repeating data like usernames, tags, or IDs.

✅ Review and Practice Questions

  1. What’s the main difference between a Set and an Array in JavaScript?

  2. What will this code log?

    let s = new Set(["x", "y", "x", "z"]);
console.log(s.size);
  3. Write a function that removes duplicate items from an array using a Set.
  4. How do you check if a Set contains a specific value?

  5. What is the output of this code?

    let ids = new Set();
ids.add(1);
ids.add(1);
ids.add("1");
console.log(ids);
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