HashSet in JavaScript (`Set`, uniqueness, Map vs dedupe)

Languages like Java and C# expose a HashSet type for unique elements and fast membership tests. JavaScript does not spell HashSet in the standard library, but hashset javascript searches almost always want the same behavior: unique values, has in average sublinear time (implementations may use hash tables internally—see MDN Set), and predictable iteration. The built-in answer is Set. For javascript hash set style key → value storage with unique keys, use Map instead.

This guide shows Set construction, add deduplication, iteration, delete / has, object identity, array dedupe, and a tiny Map contrast.

Tested on: Node.js v20.18.2. A short note after each snippet describes what you should see in the console.

Quick reference

Use this table when you are translating Java HashSet / C# HashSet mental models into JavaScript: Set for unique values, Map for unique keys with payloads, [...new Set(arr)] to dedupe arrays.

1. Create a Set (hashset in javascript)

Constructing from an iterable seeds the set in one step; size reflects unique membership.

const theSet = new Set([1, 2, 3, 4, 5]);
console.log(theSet);

You should see one line like: Set(5) { 1, 2, 3, 4, 5 }.

2. add ignores duplicates (js hashset behavior)

Repeated add with the same value is a no-op; only the first successful insertion affects order and size.

const mySet = new Set();

mySet.add("one");
mySet.add("one");
mySet.add("two");
mySet.add("three");
mySet.add("four");
mySet.add("five");

console.log(mySet);

You should see one line like: Set(5) { 'one', 'two', 'three', 'four', 'five' }.

3. Iterate in insertion order with forEach

forEach visits each member once; callback order matches insertion order, not sort order.

const mySet = new Set(["one", "two", "three", "four", "five"]);

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

You should see 5 lines of output, starting with one and ending with five.

4. delete and has

delete removes by value; has answers membership without mutating the set.

const theSet = new Set(["one", "two", "three", "four", "five"]);
console.log(theSet);

theSet.delete("five");
console.log(theSet);
console.log(theSet.has("four"));

You should see 3 lines: Set(5) { 'one', 'two', 'three', 'four', 'five' }, then Set(4) { 'one', 'two', 'three', 'four' }, then true.

5. Objects are compared by reference (hashset in js gotcha)

Primitives dedupe by value; objects dedupe only when the same reference is added twice.

Two different object literals are not equal under ===, so Set keeps both unless you delete the same reference you added.

const theSet = new Set();
theSet
  .add({ user: "jack", has: false })
  .add({ user: "Ox", has: true })
  .add({ user: "femi", has: false });

theSet.forEach((data) => {
  if (data.has === true) theSet.delete(data);
});

console.log(theSet);

You should see one line like: Set(2) { { user: 'jack', has: false }, { user: 'femi', has: false } }.

For “unique by id” or “unique by serialized shape,” use a Map keyed by id/string, or normalize before add.

6. Dedupe an array; NaN counts once

Spreading a Set built from an array is the idiomatic one-liner dedupe; NaN follows SameValueZero rules.

console.log([...new Set([1, 2, 2, 3])]);
console.log(new Set([NaN, NaN]).size);

You should see 2 lines: [ 1, 2, 3 ], then 1.

7. When people mean hash set javascript but need Map

Set is “bag of unique values.” When each record has a key and a payload, Map matches the mental model better.

const m = new Map([
  ["a", 1],
  ["b", 2],
]);
m.set("a", 3);
console.log(m.size);
console.log([...m.keys()]);

You should see 2 lines: 2, then [ 'a', 'b' ].

Summary

The main takeaway: Set for unique values and fast has, Map for unique keys with values, [...new Set(arr)] for array dedupe—mind object identity and NaN.

• Set stores each value at most once; iteration follows insertion order.
• add / has / delete / size cover the usual hash-set operations; objects use reference identity.
• Spread new Set(arr) dedupes primitives; NaN is handled once under SameValueZero.
• Use Map when each entry is a key paired with a value.

References

MDN and a Stack Overflow reference for HashSet-style collections in JavaScript.

• MDN: Set
• MDN: Map
• Stack Overflow: JavaScript equivalent to a C# HashSet