Lesson 8: Set and Map

Set: a collection of unique values

new Set(iterable), add, has, delete, size, clear

A set keeps each value once and checks membership very quickly.

const s = new Set([1, 2, 2, 3]);
s.add(4);
s.add(4);
console.log(s, s.size);
console.log(s.has(2), s.has(9));
s.delete(1);
console.log([...s]);
s.clear();
console.log(s.size);

Output:

Set(4) { 1, 2, 3, 4 } 4
true false
[ 2, 3, 4 ]
0

Removing duplicates

Spread a set back into an array.

console.log([...new Set(["a", "b", "a", "c", "b"])]);
console.log(new Set("mississippi").size);

Output:

[ 'a', 'b', 'c' ]
4

Looping over a Set

Items come out in the order they were added.

const s = new Set(["x", "y"]);
for (const v of s) console.log(v);
s.forEach((v) => console.log("each", v));
console.log([...s.entries()]);

Output:

x
y
each x
each y
[ [ 'x', 'x' ], [ 'y', 'y' ] ]

Set maths: union, intersection, difference

Built from spread and filter (newer engines also have s.union() etc.).

const a = new Set([1, 2, 3, 4]);
const b = new Set([3, 4, 5]);
console.log([...new Set([...a, ...b])]);
console.log([...a].filter((x) => b.has(x)));
console.log([...a].filter((x) => !b.has(x)));

Output:

[ 1, 2, 3, 4, 5 ]
[ 3, 4 ]
[ 1, 2 ]

Map: key-value pairs with any kind of key

new Map(), set, get, has, delete, size

Unlike an object, a Map's keys can be anything (numbers, objects), it keeps insertion order, and it has a real size.

const m = new Map();
m.set("a", 1).set(2, "two").set(true, "yes");
console.log(m.get("a"), m.get(2), m.get("zz"));
console.log(m.has(true), m.size);
m.delete("a");
console.log(m);

Output:

1 two undefined
true 3
Map(2) { 2 => 'two', true => 'yes' }

Creating from entries, and looping

Pass an array of pairs. Loop with for...of and destructuring.

const m = new Map([["Ram", 80], ["Sita", 92]]);
for (const [name, mark] of m) console.log(name, mark);
console.log([...m.keys()], [...m.values()]);
console.log(Object.fromEntries(m));

Output:

Ram 80
Sita 92
[ 'Ram', 'Sita' ] [ 80, 92 ]
{ Ram: 80, Sita: 92 }

Counting with a Map

The classic counter.

const counts = new Map();
for (const ch of "hello") counts.set(ch, (counts.get(ch) ?? 0) + 1);
console.log(counts);
console.log([...counts].sort((a, b) => b[1] - a[1])[0]);

Output:

Map(4) { 'h' => 1, 'e' => 1, 'l' => 2, 'o' => 1 }
[ 'l', 2 ]

Object keys versus Map keys

An object turns every key into text; a Map keeps the real key.

const o = {}; o[1] = "a"; o["1"] = "b";
console.log(o, Object.keys(o));
const m = new Map(); m.set(1, "a"); m.set("1", "b");
console.log(m.size);
const key = { id: 1 };
const byObject = new Map([[key, "data"]]);
console.log(byObject.get(key), byObject.get({ id: 1 }));

Output:

{ '1': 'b' } [ '1' ]
2
data undefined

WeakMap and WeakSet

Like Map and Set but they hold objects weakly: when nothing else refers to an object, it can be garbage-collected. They cannot be looped over. Use them to attach private data to objects without causing memory leaks.

WeakMap example

Keys must be objects.

const secret = new WeakMap();
const user = { name: "Sita" };
secret.set(user, "token-123");
console.log(secret.get(user), secret.has({}));

Output:

token-123 false

Choosing

Try it yourself

1. Find the letters two words share, using sets.
2. Check whether an array has any duplicate with a Set and size.
3. Use a Map to cache the result of a slow function.

Next lesson: functions.