Hash Set

What it is

A hash set stores unique values.

Unlike a hash map, the point here is not associating a key with a value.

The point is answering quickly:

“have I seen this value already?”

In JavaScript

Set is the direct implementation:

const seen = new Set<number>()

seen.add(3)
seen.has(3)

If you only care about existence, Set is usually simpler than Map.

When to use it

Use Set when you need to:

• remove duplicates
• track whether something has appeared
• prevent repeated processing

Better question

Before using it, ask:

1. do I only need existence, or also an associated value?
2. am I avoiding duplicates or counting frequency?
3. is the extra memory worth the faster lookup?