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Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Map/size below:

Map.prototype.size - JavaScript | MDN

Baseline Widely available

Map 实例的 size 访问器属性返回此 map 中元素的数量。

const map1 = new Map();

map1.set("a", "alpha");
map1.set("b", "beta");
map1.set("g", "gamma");

console.log(map1.size);
// Expected output: 3

size 的值是一个整数，表示 Map 对象有多少个键值对。size 的设置访问器函数是 undefined；你无法更改此属性的值。

const myMap = new Map();
myMap.set("a", "alpha");
myMap.set("b", "beta");
myMap.set("g", "gamma");

console.log(myMap.size); // 3

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