JavaScript — Sets

A Set holds no duplicates

const ids = new Set([1, 2, 2, 3, 3, 3]);
console.log(ids);        // Set(3) {1, 2, 3}

Adding an existing value does nothing:

ids.add(1);      // ignored — already present
ids.add(4);      // added
ids.has(2);      // true    membership check
ids.delete(2);   // true (was there)
ids.size;        // 3

The killer use: deduplicating arrays

const tags = ["css", "js", "css", "html", "js"];

const unique = [...new Set(tags)];
// ["css", "js", "html"]

One line replaces the manual filter-and-check dance. Works on primitives by value:

[...new Set("mississippi")].join("");   // "mispi"

Why not just includes()?

arr.includes(x);    // scans the array — O(n)
set.has(x);         // hash lookup — effectively O(1)

For "have I seen this before?" checks inside loops, Sets scale where includes crawls:

const seen = new Set();
for (const row of bigData) {
    if (seen.has(row.id)) continue;   // instant skip
    seen.add(row.id);
    process(row);
}

Iteration preserves insertion order

for (const id of ids) console.log(id);
[...ids];              // back to array
ids.forEach(v => …);   // also available

Set algebra

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

// union — everything in either
const union = new Set([...a, ...b]);            // {1,2,3,4}

// intersection — in both
const both = [...a].filter(x => b.has(x));      // [3]

// difference — in a but not b
const onlyA = [...a].filter(x => !b.has(x));    // [1,2]

WeakSet (one paragraph)

WeakSet holds only objects and lets them be garbage-collected when unused elsewhere. Niche: marking objects as processed without preventing cleanup.

Gotchas: no index access (set[0] meaningless — convert to array first); object equality is by reference, so {a:1} added twice stores twice.

Mini Practice

1. Deduplicate an array of names three ways: Set, filter+indexOf, reduce
2. Time includes vs has over 100k lookups (console.time)
3. Build "mutual friends": intersection of two friend sets
4. Track unique site visitors per day with add/size
5. Union two tag lists into a sorted unique array

Next: maps →