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Map - JavaScript | MDN

Baseline Widely available *

Map 是保存了鍵值對（key-value pairs）的物件。任何值（包括物件及基本型別（primitive）值）都可以作為鍵或值。

iterable

為一個陣列或其他元素成鍵值對的可迭代物件（有兩個元素的陣列，例如 [[ 1, 'one' ],[ 2, 'two' ]]）。每一個鍵值對都會被加入至新的 Map；null 會被視為 undefined。

一個 Map 物件會根據元素的新增順序走訪自身的所有元素 — for...of 迴圈會在每次迭代回傳一個 [key, value] 陣列。

鍵相等是基於 SameValueZero 的演算法：NaN 被認為與 NaN 相同（即使 NaN !== NaN）並且根據 === 運算符的語義，所有其他值都被認為相等。在目前的 ECMAScript 規範中，-0 和 +0 被認為是相等的，儘管在早期的草案中並非如此。詳細的內容請參閱 瀏覽器相容性 表中的 "Value equality for -0 and 0"。

Object 和 Map 類似。兩者都允許你設置對應的鍵值對，檢索這些值，刪除鍵，並檢測是否有什麼存儲在鍵中。正因為如此（也因為沒有內置的替代品），Object 在歷史上一直被當作 Map 使用；然而在某些情況下，使用 Map 有一些重要的不同之處:

• Object 的鍵是 字串 和 Symbol，而它們在 Map 中可以是任意的資料型態，包括函數，對象以及原始的資料型態。
• 你可以使用 size 屬性輕鬆地獲得 Map 的大小，而 Object 中的屬性數量必須手動確認。
• Map 是可迭代（iterable）的，因此可以直接迭代，而在 Object 上迭代則需要以某種方式獲取其鍵並對其進行迭代。
• Object 有一個原型，所以如果不小心，映射中有一些默認鍵可能與鍵發生衝突。從 ES5 開始，這可以通過使用 map = Object.create(null) 來繞過這個問題，但是很少這樣做。
• 在涉及頻繁添加和刪除鍵值對的場景中，Map 可能表現得更好。

Map.length

length屬性的值為 0 要計算 Map 中有多少元素，可以使用 Map.prototype.size。

Map[Symbol.species]

用於創建派生物件的構造函數。

Map.prototype

表示 Map 構造函數的原型，允許對所有的 Map 物件添加屬性

所有的 Map 實例都繼承自 Map.prototype.

Map.prototype[Symbol.toStringTag]

The initial value of the Symbol.toStringTag property is the string "Map". This property is used in Object.prototype.toString().

Map.prototype.size

Returns the number of key/value pairs in the Map object.

Map.prototype.clear()

Removes all key-value pairs from the Map object.

Map.prototype.delete()

Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. map.has(key) will return false afterwards.

Map.prototype.get()

Returns the value associated to the passed key, or undefined if there is none.

Map.prototype.has()

Returns a boolean indicating whether a value has been associated with the passed key in the Map object or not.

Map.prototype.set()

Sets the value for the passed key in the Map object. Returns the Map object.

Map.prototype[Symbol.iterator]()

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Map.prototype.keys()

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

Map.prototype.values()

Returns a new Iterator object that contains the values for each element in the Map object in insertion order.

Map.prototype.entries()

Returns a new Iterator object that contains a two-member array of [key, value] for each element in the Map object in insertion order.

Map.prototype.forEach()

Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.

var myMap = new Map();

var keyString = "a string",
  keyObj = {},
  keyFunc = function () {};

// setting the values
myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, "value associated with keyObj");
myMap.set(keyFunc, "value associated with keyFunc");

myMap.size; // 3

// getting the values
myMap.get(keyString); // "value associated with 'a string'"
myMap.get(keyObj); // "value associated with keyObj"
myMap.get(keyFunc); // "value associated with keyFunc"

myMap.get("a string"); // "value associated with 'a string'"
// because keyString === 'a string'
myMap.get({}); // undefined, because keyObj !== {}
myMap.get(function () {}); // undefined, because keyFunc !== function () {}

NaN 同樣可以作為 Map 的 key，雖然每個 NaN 都不等於自己本身，下面的例子是有效的，因為 NaN 無法區分彼此。

var myMap = new Map();
myMap.set(NaN, "not a number");

myMap.get(NaN); // "not a number"

var otherNaN = Number("foo");
myMap.get(otherNaN); // "not a number"

Map 可以使用 for..of 迴圈進行迭代：

var myMap = new Map();
myMap.set(0, "zero");
myMap.set(1, "one");
for (var [key, value] of myMap) {
  console.log(key + " = " + value);
}
// 0 = zero
// 1 = one

for (var key of myMap.keys()) {
  console.log(key);
}
// 0
// 1

for (var value of myMap.values()) {
  console.log(value);
}
// zero
// one

for (var [key, value] of myMap.entries()) {
  console.log(key + " = " + value);
}
// 0 = zero
// 1 = one

Map 可以使用 forEach() 方法進行迭代：

myMap.forEach(function (value, key) {
  console.log(key + " = " + value);
});
// Will show 2 logs; first with "0 = zero" and second with "1 = one"

var kvArray = [
  ["key1", "value1"],
  ["key2", "value2"],
];

// Use the regular Map constructor to transform a 2D key-value Array into a map
var myMap = new Map(kvArray);

myMap.get("key1"); // returns "value1"

// Use the Array.from function to transform a map into a 2D key-value Array
console.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray

// Or use the keys or values iterators and convert them to an array
console.log(Array.from(myMap.keys())); // Will show ["key1", "key2"]

就像 Array 一樣，Map 可以被複製:

var original = new Map([[1, "one"]]);

var clone = new Map(original);

console.log(clone.get(1)); // one
console.log(original === clone); // false. Useful for shallow comparison

請記住，數據本身並非克隆的。

Map 可以被合併，保持鍵的唯一性:

var first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

var second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// Merge two maps. The last repeated key wins.
// Spread operator essentially converts a Map to an Array
var merged = new Map([...first, ...second]);

console.log(merged.get(1)); // uno
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

Map 也可以跟 Array 合併：

var first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

var second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// Merge maps with an array. The last repeated key wins.
var merged = new Map([...first, ...second, [1, "eins"]]);

console.log(merged.get(1)); // eins
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

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