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Map.prototype.size - JavaScript | MDN
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Map.prototype.size
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
size は Map インスタンスのアクセサープロパティで、このマップにある要素の数を返します。
試してみましょう
const map = new Map();
map.set("a", "alpha");
map.set("b", "beta");
map.set("g", "gamma");
console.log(map.size);
// 予想される結果: 3
解説
size の値は Map オブジェクトにいくつの項目があるかを表す整数値です。 size の set アクセサー関数は undefined です。このプロパティを変更することはできません。
例
size の使用
js
const myMap = new Map();
myMap.set("a", "alpha");
myMap.set("b", "beta");
myMap.set("g", "gamma");
console.log(myMap.size); // 3
仕様書
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-get-map.prototype.size |
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