📘Map

🚧 under construction

breadcrumb 🚧

⭐️ ES6 (2015)

🚧

⭐️ 重點

Map

• is iterable.

• is a data structure.

• is a better alternative for associated array.

👥 相關

• Set

📘 手冊

• Map ⟩ Objects vs. Maps

📗 參考

• JS.info ⟩ Map & Set

• JavaScript: The Definitive Guide ⟩ 11.1.2

雖然 Object 與 Map 都是 JS 用來實現 Dictionary 的方法，但它們存在基本的不同點。

Maps vs. Objects

feature	Map	Object
iterable	✅ use [...map] to convert into Array. ✅ for(const [key, value] of map){...} ✅ for(const key of map.keys()){...} ✅ for(const value of map.values()){...} ✅ map.forEach((value, key) => {...})	❌ does not implement iteration protocol. ❌ not directly iterable using for...of. ⭕ can use for...in to iterate over enumerable properties.
keys	can be any value (including functions, objects, or any primitive).	must be either String or Symbol.
size	✅ number of items in a Map	❌ no such property

Map ⇔ Array

// Array => Map
const map = new Map([['key1', 'value1'], ['key2', 'value2']])

// Map => Array<[Key, Value]>
[...map]        // [['key1', 'value1'], ['key2', 'value2']]

// Map => Array<Value>
[...map.values()]

Other Topics

• Cloning and merging Maps