JavaScript

This page explains JavaScript Map, a collection of key-value pairs that keeps insertion order.

Goal: understand Map keys and values, then use set(), get(), has(), and delete() correctly.

Map

A Map associates a key with a value. Unlike ordinary object properties, Map keys can be values of many types, including numbers and objects. Entries are iterated in insertion order.

Basic operations

const scores = new Map();

scores.set("Aki", 80);
scores.set("Ren", 95);

console.log(scores.get("Aki")); // 80
console.log(scores.has("Ren")); // true
console.log(scores.size);        // 2

scores.delete("Aki");

• set(key, value) adds or replaces an entry.
• get(key) returns the value for a key, or undefined if it is missing.
• has(key) checks whether a key exists.
• delete(key) removes one entry.
• clear() removes every entry.

Object keys are allowed

const settings = new Map();
const user = { id: 1 };

settings.set(user, { theme: "dark" });
console.log(settings.get(user).theme); // dark

Map compares object keys by identity. The exact same object reference must be used to retrieve the entry.

Map or object?

Use a Map when keys can be arbitrary values, insertion order matters, or frequent key-based updates are central to the design. Use an ordinary object when you are modeling a simple record with known string keys and JSON-like data.

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