Map and Set
Beyond objects and arrays, JavaScript offers two specialized structures.
Map: key-value pairs
A Map associates keys with values. Unlike a normal object,
its keys can be of any type (even objects or functions) and
it remembers the insertion order.
const stock = new Map();
stock.set("apples", 50);
stock.set("pears", 30);
console.log(stock.get("apples")); // 50
console.log(stock.has("pears")); // true
console.log(stock.size); // 2
stock.delete("pears");
You can iterate over it easily:
for (const [fruit, amount] of stock) {
console.log(fruit, amount);
}
Set: unique values
A Set is a collection of values without duplicates.
const tags = new Set(["js", "web", "js"]);
console.log(tags.size); // 2 (the duplicate is ignored)
tags.add("css");
console.log(tags.has("web")); // true
A common trick: remove duplicates from an array.
const unique = [...new Set([1, 1, 2, 3, 3])]; // [1, 2, 3]
When to use each one?
Map: when you need a dictionary with non-string keys or with order.Set: when you only care about membership and uniqueness.
Examples
Counting frequencies with Map
const words = ["sun", "sea", "sun", "light", "sea", "sun"];
const count = new Map();
for (const w of words) {
count.set(w, (count.get(w) || 0) + 1);
}
console.log([...count.entries()]);Union of two sets with Set
const a = new Set([1, 2, 3]);
const b = new Set([3, 4, 5]);
const union = new Set([...a, ...b]);
console.log([...union]);