Set

JavaScript · Reference cheat sheet

📋 Overview

Set stores unique values using SameValueZero equality. Insertion order is preserved. Use sets for membership tests, deduplication, and set-algebra helpers.

🔧 Core concepts

Create: new Set(), new Set(iterable).
Mutate: add, delete, clear, has.
Size: set.size.
Iterate: keys() / values() (same), entries() → [v, v], forEach, for...of.
Equality: NaN equals NaN; objects by reference.

const s = new Set([1, 2, 2, 3]);
s.size; // 3
s.has(2); // true

💡 Examples

// Deduplicate array
const unique = [...new Set(items)];

// Membership
const allowed = new Set(["read", "write", "admin"]);
if (allowed.has(role)) { /* ... */ }

// Union / intersection / difference (ES2025 methods where available)
function union(a, b) {
  return new Set([...a, ...b]);
}
function intersection(a, b) {
  return new Set([...a].filter((x) => b.has(x)));
}
function difference(a, b) {
  return new Set([...a].filter((x) => !b.has(x)));
}

// Native (modern engines)
// a.union(b); a.intersection(b); a.difference(b); a.symmetricDifference(b);

// Toggle membership
function toggle(set, value) {
  if (set.has(value)) set.delete(value);
  else set.add(value);
  return set;
}

// WeakSet — objects only, not iterable
const seen = new WeakSet();
function visit(node) {
  if (seen.has(node)) return;
  seen.add(node);
}

for (const v of s) console.log(v);

⚠️ Pitfalls

Object uniqueness is by reference: new Set([\{a:1\},\{a:1\}]).size === 2.
JSON.stringify(set) → "\{\}" — convert with [...set] first.
set[0] does not work — use iteration or convert to array.
WeakSet cannot hold primitives and is not enumerable.
Mutating while iterating is allowed but can be confusing — prefer snapshots.

map.md — Map collections
arrays.md — dedupe patterns
iteration.md — looping sets
for_of.md — for...of
objects.md — reference equality
json.md — serialization limits

Set

Set

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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