`Set` オブジェクト

 1// Create a Set and demonstrate add, has, delete, and size
 2const s = new Set();
 3
 4s.add(1);
 5s.add(2);
 6s.add(2); // duplicate, ignored
 7
 8console.log(s.has(1)); // true
 9console.log(s.has(3)); // false
10
11s.delete(2);
12console.log(s.size); // 1
13
14console.log([...s]); // [1]

 1// Iterate a Set with for...of and forEach
 2const s = new Set(['a', 'b', 'c']);
 3
 4for (const v of s) {
 5  console.log('for...of:', v);
 6}
 7
 8s.forEach((value, sameValue, setRef) => {
 9  // Note: second arg is same as first for Set API to match Map signature
10  console.log('forEach:', value);
11});

1// Deduplicate an array using Set
2const arr = [1, 2, 2, 3, 3, 3];
3const deduped = [...new Set(arr)];
4console.log(deduped); // [1, 2, 3]
5
6// Convert a Set to an array using Array.from
7const s = new Set([4, 5, 6]);
8const arrFromSet = Array.from(s);
9console.log(arrFromSet); // [4, 5, 6]

 1// Objects are compared by reference in a Set
 2const obj1 = { x: 1 };
 3const obj2 = { x: 1 };
 4
 5const s = new Set();
 6s.add(obj1);
 7s.add(obj2);
 8
 9console.log(s.size); // 2 (different references)
10console.log(s.has(obj1)); // true
11console.log(s.has({ x: 1 })); // false (different object)

 1// NaN and zero behavior in Set
 2const s = new Set();
 3
 4s.add(NaN);
 5s.add(NaN);
 6console.log(s.size); // 1 (NaN considered the same)
 7
 8s.add(+0);
 9s.add(-0);
10console.log(s.size); // still 2 (NaN + 0)
11console.log([...s]); // [NaN, 0] (order may vary but only one zero)

 1// Set operations: union, intersection, difference
 2function union(a, b) {
 3  return new Set([...a, ...b]);
 4}
 5
 6function intersection(a, b) {
 7  return new Set([...a].filter(x => b.has(x)));
 8}
 9
10function difference(a, b) {
11  return new Set([...a].filter(x => !b.has(x)));
12}
13
14// Demo
15const A = new Set([1, 2, 3]);
16const B = new Set([3, 4, 5]);
17
18console.log('union', [...union(A, B)]); // [1,2,3,4,5]
19console.log('intersection', [...intersection(A, B)]); // [3]
20console.log('difference A\\B', [...difference(A, B)]); // [1,2]

 1// Find added and removed items between two arrays
 2function diffArrays(oldArr, newArr) {
 3  const oldSet = new Set(oldArr);
 4  const newSet = new Set(newArr);
 5
 6  const added = [...newSet].filter(x => !oldSet.has(x));
 7  const removed = [...oldSet].filter(x => !newSet.has(x));
 8
 9  return { added, removed };
10}
11
12const oldList = [1, 2, 3];
13const newList = [2, 3, 4, 5];
14
15console.log(diffArrays(oldList, newList));
16// { added: [4,5], removed: [1] }

1// WeakSet basics (objects only, not iterable)
2const ws = new WeakSet();
3let obj = { id: 1 };
4ws.add(obj);
5
6console.log(ws.has(obj)); // true
7
8obj = null; // Now the object is eligible for GC; WeakSet won't prevent collection

 1// WeakSet basics (objects only, not iterable)
 2const ws = new WeakSet();
 3
 4// --- Only objects can be added ---
 5try {
 6	ws.add(1); // number
 7} catch (e) {
 8	console.log("Error: WeakSet can only store objects. Adding a number is not allowed.");
 9}
10
11try {
12	ws.add("text"); // string
13} catch (e) {
14	console.log("Error: WeakSet can only store objects. Adding a string is not allowed.");
15}
16
17// --- WeakSet is not iterable ---
18try {
19	for (const value of ws) {
20		console.log(value);
21	}
22} catch (e) {
23	console.log("Error: WeakSet is not iterable. You cannot use for...of to loop over its elements.");
24}
25
26// --- Cannot convert to array ---
27try {
28	console.log([...ws]);
29} catch (e) {
30	console.log("Error: WeakSet cannot be converted to an array because it does not support iteration.");
31}
32
33// The object becomes eligible for garbage collection
34let obj = { id: 1 };
35ws.add(obj);
36obj = null;