JavaScript Map 和 Set

结论

  • Map:存放键值对,区别于 Object,键可以是任何值。
  • Set:存放不重复的值

Map

存储键值对,读取时与插入顺序一致。


var map = new Map([[1, "1"], [3, "3"], [2, "2"]]);
map.set("foo", "bar");

for (const [key, val] of map) {
  console.log(key, val);
}

输出:

1 '1'
3 '3'
2 '2'
foo bar

任何值,对象或原始值,都可作为 Map 的键。

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 () {}

相比 Object,除了对可作为键的值没要求外,Map 自带遍历器(iterator),可对其使用 for of 语句。

同时还自带一些便捷的属性和方法,比如 sizeclear()

Set

存储唯一的值,对于重复的值会被忽略。

示例:

var obj = { a: 1, b: 2 };
var set = new Set([1, 2, 2, "foo"]);
set.add(obj);
set.add(obj);

console.log("size:", set.size);
console.log(set.has(2));
console.log(set.has(obj));

for (const val of set) {
  console.log(val);
}

输出:

4
true
true
1
2
foo
{ a: 1, b: 2 }

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