映射键值对：Map()

要义盘点

• 创建映射
• 使用自定义迭代器初始化映射
• set方法添加键值对
• size属性查询键值对数量
• get与has方法查询指定键的值
• delete与clear方法删除键值对
• 键名为NaN或者键名为+/-0时可能会报错
• 三种返回可迭代对象的情况

创建映射并初始化

new Map():

        let m = new Map([
            ['key1', 'val1'],
            ['key2', 'val2'],
            ['key3', 'val3']
        ]);

使用自定义迭代器初始化映射

        const m2 = new Map({
            [Symbol.iterator]: function* () {
                yield ['key1', 'val1'];
                yield ['key2', 'val2'];
                yield ['key3', 'val3'];
            }
        });

添加键值对：使用set()方法

        let m = new Map().set('key1', 'val1').set('key2', 'val2');
        console.log(m.size); // 2

查询键值对数量：size

        let m = new Map([
            ['key1', 'val1'],
            ['key2', 'val2'],
            ['key3', 'val3']
        ]);

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

查询：get()与has()方法

        let m = new Map([
            ['key1', 'val1'],
            ['key2', 'val2'],
            ['key3', 'val3']
        ]);

        console.log(m.has('key1')); // true
        console.log(m.get('key2')); // val2

删除：delete()与clear()方法

        let m = new Map([
            ['key1', 'val1'],
            ['key2', 'val2'],
            ['key3', 'val3']
        ]);

        m.delete('key2');
        console.log(m.size); // 2

        console.log(m.has('key2')); // false
        console.log(m.get('key2')); // undefined

        m.clear();
        console.log(m.size); // 0

键名为NaN或者键名为+/-0时可能会报错

        let key1 = 0 / ""; // NaN
        let key2 = 0 / ""; // NaN
        console.log(key1 === key2); // false

        let m = new Map().set(key1, 'val1');
        console.log(m.get(key2)); // val1

        let key1 = +0; // NaN
        let key2 = -0; // NaN
        console.log(key1 === key2); // true

        let m = new Map().set(key1, 'val1');
        console.log(m.get(key2)); // val1

返回可迭代对象：使用entries()方法

        let m = new Map().set('key1', 'val1').set('key2', 'val2').set('key3', 'val3');
        console.log(m.entries === m[Symbol.iterator]); //true
        for (let prop of m.entries()) {
            console.log(prop);
            console.log(Array.isArray(prop)); //true
        }
        // ['key1','val1']
        // ['key2','val2']
        // ['key3','val3']

返回可迭代对象：使用数组扩展运算符

        let m = new Map().set('key1', 'val1').set('key2', 'val2').set('key3', 'val3');
        for (let prop of [...m]) {
            console.log(prop);
            console.log(Array.isArray(prop)); //true

        }
        // ['key1','val1']
        // ['key2','val2']
        // ['key3','val3']

返回键或值迭代对象：使用keys()和values()方法

        let m = new Map().set('key1', 'val1').set('key2', 'val2').set('key3', 'val3');
        for (let prop of m.keys()) {
            console.log(prop);
        }
        // key1 
        // key2
        // key3

        let m = new Map().set('key1', 'val1').set('key2', 'val2').set('key3', 'val3');
        for (let prop of m.values()) {
            console.log(prop);
        }
        // val1 
        // val2
        // val3