JS Sets

JS Sets

A Set is a collection of unique values. If you add the same value multiple times, it will only exist once.

const set = new Set();

set.add("a");
set.add("b");
set.add("b"); // duplicate ignored

console.log(set.size); // 2
console.log(set.has("a")); // true

Sets are useful for removing duplicates from arrays.

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

console.log(unique); // [1,2,3]

JS Set Methods

Common Set methods include add, has, delete, clear, and forEach.

const letters = new Set(["a", "b", "c"]);

letters.add("d");
console.log(letters.has("c")); // true

letters.delete("b");
console.log(letters.size); // 3

letters.forEach((value) => {
  console.log(value);
});

You can iterate over a Set using for...of.

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

for (const c of colors) {
  console.log(c);
}

JS Set Logic

Sets make it easy to implement common set operations like union, intersection, and difference.

const a = new Set([1, 2, 3]);
const b = new Set([3, 4, 5]);

// Union
const union = new Set([...a, ...b]);

// Intersection
const intersection = new Set([...a].filter(x => b.has(x)));

// Difference (A - B)
const difference = new Set([...a].filter(x => !b.has(x)));

console.log([...union]);        // [1,2,3,4,5]
console.log([...intersection]); // [3]
console.log([...difference]);   // [1,2]

JS Set WeakSet

WeakSet is similar to Set but it only stores objects and does not prevent them from being garbage-collected.

WeakSet is not iterable and has no size. It is mainly used for tracking objects privately.

const visited = new WeakSet();

let user = { id: 1 };
visited.add(user);

console.log(visited.has(user)); // true

user = null; // object can be garbage-collected

JS Set Reference

Sets are ideal for uniqueness, fast lookups, and set operations. Key methods include add(), has(), delete(), clear(), and iteration via for...of.

Next Step

Continue with JS Maps to store key-value pairs with any key type.