Summary - Map and Set

Build a word counter app using Map and Set.

Summary - Map and Set

In this group you learned Map for an ordered collection that accepts any key type, and Set for a collection of unique values. You manage both with set/get/has/delete and size.

WeakMap and WeakSet hold object references weakly so the garbage collector can reclaim them; they cannot be iterated. Map is faster than Object for frequent add/remove of key-value pairs and guarantees insertion order. Now you will combine them in a word counter.

Exercise Word counter app

Split a sentence string by spaces. Count how many times each word appears in a Map called counts; use counts.get(word) || 0 to read the current value. Then collect the unique words into a Set called uniqueWords and log how many unique words there are with console.log.

Show example solution

const sentence = "apple pear apple banana pear apple";
const words = sentence.split(" ");
const counts = new Map();
const uniqueWords = new Set();
for (const word of words) {
  counts.set(word, (counts.get(word) || 0) + 1);
  uniqueWords.add(word);
}
console.log(uniqueWords.size);

Test yourself

Which scenario is the best fit for Set?
- Attach temporary metadata to an object
- Store a user's roles by id
- Find unique cities from a list
- Hold fixed-shape user data
Show answer

Find unique cities from a list — Correct. Set filters out duplicates automatically; ideal for unique cities.

Complete the word counter in one line

const counts = new Map();
const word = "apple";
counts.set(word, (counts.get(word) || 0) + ____);
console.log(counts.get(word));

Show answer

const counts = new Map();
const word = "apple";
counts.set(word, (counts.get(word) || 0) + 1);
console.log(counts.get(word));

Match the collections

Map

Key-value, ordered
Set

Unique values
WeakMap

Object keys, GC friendly
Object

Fixed shape, prototype chain

Next lesson: The iterator protocol

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