Sets & Maps
Store unique values with Set and keyed data with Map.
Syntax
new Set([1, 2, 2])
new Map()ES6 added two useful collection types:
- Set — a collection of unique values. Adding a duplicate has no effect.
- Map — key/value pairs where keys can be any type, with a reliable size and order.
Example
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When to use it
- Deduplicate an array of user-selected tags using a Set before saving them to the database.
- Cache API responses in a Map keyed by URL so repeated requests return instantly.
- Count how many times each word appears in a text string using a Map as a frequency table.
More examples
Set for unique values
Creates a Set from an array to remove duplicates, then spreads it back into an array.
const rawTags = ["js", "css", "js", "html", "css"];
const unique = new Set(rawTags);
console.log([...unique]); // ["js", "css", "html"]Map as a keyed cache
Uses a Map to cache results by URL so repeated calls skip the simulated fetch.
const cache = new Map();
function fetchWithCache(url) {
if (cache.has(url)) return cache.get(url);
const data = `data for ${url}`; // simulated fetch
cache.set(url, data);
return data;
}
console.log(fetchWithCache("/api/users"));
console.log(fetchWithCache("/api/users")); // returns cachedMap as word frequency counter
Builds a frequency table using a Map where each word is a key and its count is the value.
const words = ["the", "cat", "sat", "on", "the", "mat", "the"];
const freq = new Map();
for (const w of words) {
freq.set(w, (freq.get(w) ?? 0) + 1);
}
console.log(freq.get("the")); // 3
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