พื้นฐาน Map และ Set

เหตุใดจึงใช้แมปและเซต

แมป: set/get/has

// Map: key→value with any key type
const userToScore = new Map();

// set values
userToScore.set("ayla", 10);
userToScore.set("mina", 15);

// read values
console.log("ayla:", userToScore.get("ayla"));
console.log("has lena?", userToScore.has("lena"));

// size
console.log("size:", userToScore.size);

เซต: ค่าไม่ซ้ำกัน

// Set: unique values
const seen = new Set();

// add values (duplicates are ignored)
seen.add("red");
seen.add("blue");
seen.add("red"); // ignored

console.log("size:", seen.size);
console.log("has red?", seen.has("red"));

seen.delete("blue");
console.log("after delete:", seen.size);

เหตุใดจึงเลือกแมปแทนออบเจ็กต์

// Map can use objects as keys directly
const key1 = { id: 1 };
const key2 = { id: 1 }; // different reference

const map = new Map();
map.set(key1, "A");

console.log("get key1:", map.get(key1));
console.log("get key2:", map.get(key2)); // undefined (different object)

const obj = {};
obj[key1] = "B"; // coerces key to "[object Object]"
console.log("object key:", obj["[object Object]"]);

การแปลงข้อมูล

// Unique array values via Set
const nums = [2, 2, 3, 4, 3];
const unique = Array.from(new Set(nums));
console.log("unique:", unique);

// Map from pairs; back to pairs
const pairs = [["a", 1], ["b", 2]];
const m = new Map(pairs);
console.log("get a:", m.get("a"));
console.log("entries:", Array.from(m.entries()));

รูปแบบที่ควรจำ

แบบทดสอบค่าไม่ซ้ำในเซต

ทบทวน