What is Set in TypeScript?

TypeScriptJunior~2 min read

1. What `Set` is

Set<T> is a collection of values of type T, where each value occurs only once.

Example:

javascript

const numbers = new Set<number>([1, 2, 3, 3, 2]);
console.log(numbers); // Set(3) { 1, 2, 3 }

All duplicates were removed automatically. The order of elements is kept in insertion order.

2. Declaring a `Set` with a type

javascript

const names: Set<string> = new Set(["Tim", "Max", "Oleh"]);

Each element will be strictly string. Trying to add a number causes an error:

javascript

names.add("Bob"); // OK
names.add(123);    // Error: Argument of type 'number' is not assignable to parameter of type 'string'.

3. Main `Set` methods

Method	What it does	Example
`.add(value)`	Adds an element	`set.add(5)`
`.delete(value)`	Removes an element	`set.delete(2)`
`.has(value)`	Checks presence	`set.has(3)` -> `true`
`.clear()`	Removes all elements	`set.clear()`
`.size`	Number of elements	`set.size` -> `3`
`.forEach(fn)`	Iterates elements	`set.forEach(v => console.log(v))`

Usage example:

javascript

const ids = new Set<number>();

ids.add(10);
ids.add(20);
ids.add(10); // duplicate - ignored

console.log(ids.has(10)); // true
console.log(ids.size);    // 2

4. Iterating a Set

Set is an iterable structure, so you can use:

javascript

for (const value of ids) {
  console.log(value);
}

ids.forEach(v => console.log(v));

or turn it into an array:

javascript

const arr = [...ids]; // [10, 20]

5. Differences between `Set` and an array

Property	`Array`	`Set`
Duplicates	allowed	not allowed
Presence check	slow (`arr.includes()`)	fast (`set.has()`)
Element order	preserved	preserved
Access by index	yes (`arr[0]`)	no
Adding	`push()`	`add()`
Removing	`splice()`	`delete()`
Size	`arr.length`	`set.size`
Type	`T[]`	`Set<T>`

Comparison example

javascript

const arr = [1, 2, 2, 3];
const set = new Set(arr);

console.log(arr.length); // 4
console.log(set.size);   // 3

Set automatically removed the duplicates.

6. Converting between `Array` and `Set`

From an array to a Set

javascript

const arr = [1, 2, 3, 3];
const unique = new Set(arr); // Set(3) {1, 2, 3}

From a Set to an array

javascript

const arr2 = Array.from(unique); // [1, 2, 3]

javascript

const arr3 = [...unique]; // [1, 2, 3]

7. Typing a Set with objects

You can store more than just primitives:

javascript

interface User {
  id: number;
  name: string;
}

const users = new Set<User>();

users.add({ id: 1, name: "Tim" });
users.add({ id: 1, name: "Tim" }); // both remain, because they are different references

Set checks uniqueness by reference, not by an object's content. That is, {id:1,name:"Tim"} and {id:1,name:"Tim"} are different elements.

8. A practical usage example

Removing duplicates from an array

javascript

const numbers = [1, 2, 2, 3, 4, 3];
const unique = [...new Set(numbers)];
console.log(unique); // [1, 2, 3, 4]

Counting unique values

javascript

function countUnique<T>(arr: T[]): number {
  return new Set(arr).size;
}

console.log(countUnique(["a", "b", "a", "c"])); // 3

Summary

Set is a collection of unique values, with fast lookup and iteration.

The main differences from an array:

does not store duplicates;

has no indexes;

checks for the presence of elements faster;

stores values in insertion order.

The basic type:

javascript
const set: Set<number> = new Set([1, 2, 3]);

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