Map类似于Object，但是和Object不同的地方是Map的key可以是任意值，常用的Map的api
- new Map() 创建Map
- map.set(key, value) 存储值
- map.get(key) 取值
- map.has(key) 判断key是否存在
- map.delete(key) 删除key
- map.clear() 清空
- map.size 获取元素的数量
例如：
let map = new Map(); map.set('1', 'str1'); // a string key map.set(1, 'num1'); // a numeric key map.set(true, 'bool1'); // a boolean key // remember the regular Object? it would convert keys to string // Map keeps the type, so these two are different: alert( map.get(1) ); // 'num1' alert( map.get('1') ); // 'str1' alert( map.size ); // 3
迭代Map
let recipeMap = new Map([ ['cucumber', 500], ['tomatoes', 350], ['onion', 50] ]); // iterate over keys (vegetables) for (let vegetable of recipeMap.keys()) { alert(vegetable); // cucumber, tomatoes, onion } // iterate over values (amounts) for (let amount of recipeMap.values()) { alert(amount); // 500, 350, 50 } // iterate over [key, value] entries for (let entry of recipeMap) { // the same as of recipeMap.entries() alert(entry); // cucumber,500 (and so on) } recipeMap.forEach( (value, key, map) => { alert(`${key}: ${value}`); // cucumber: 500 etc });
Map的定义，可以使用数组进行初始化
// array of [key, value] pairs let map = new Map([ ['1', 'str1'], [1, 'num1'], [true, 'bool1'] ]); alert( map.get('1') ); // str1
可以使用对象进行初始化
let obj = { name: "John", age: 30 }; let map = new Map(Object.entries(obj)); alert( map.get('name') ); // John
可以使用Object.fromEntries将Map转换为Object
let map = new Map(); map.set('banana', 1); map.set('orange', 2); map.set('meat', 4); let obj = Object.fromEntries(map.entries()); // make a plain object (*) // done! // obj = { banana: 1, orange: 2, meat: 4 } alert(obj.orange); // 2
Set，没有key，没有重复的值，可以使用的api
- new Set(iterable) 创建set
- set.add(value) 添加值
- set.delete(value) 删除值
- set.has(value) 值是否存在
- set.clear() 清空值
- set.size 返回元素的个数
示例：
let set = new Set(); let john = { name: "John" }; let pete = { name: "Pete" }; let mary = { name: "Mary" }; // visits, some users come multiple times set.add(john); set.add(pete); set.add(mary); set.add(john); set.add(mary); // set keeps only unique values alert( set.size ); // 3 for (let user of set) { alert(user.name); // John (then Pete and Mary) }
迭代set的方法
let set = new Set(["oranges", "apples", "bananas"]); for (let value of set) alert(value); // the same with forEach: set.forEach((value, valueAgain, set) => { alert(value); });
