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javascripts中的map

 2 years ago
source link: https://zhangrr.github.io/posts/20220523-javascripts_map/
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Javascripts中的map

2022-05-23 2 分钟阅读

Javascripts 中的 map

说到map，如果我们有循环的话，不停遍历，会把时间搞成 O(n)。如果有map，时间就变成了O(1)，所以还是相当有效率的。

map 是 ES2015 中新增加的，是一种新的 Object 类型，允许存放各种 key-value 对。

map 最大的特性是，key 可以是任何类型，包括 object 和 function；可以调用 size 得到 map 的大小；迭代的时候，是严格按照添加的顺序进行迭代的。

map 的方法如下： set, get, size, has, delete, clear:

let things = new Map();

const myFunc = () => '🍕';

things.set('🚗', 'Car');
things.set('🏠', 'House');
things.set('✈️', 'Airplane');
things.set(myFunc, '😄 Key is a function!');

things.size; // 4

things.has('🚗'); // true

things.has(myFunc) // true
things.has(() => '🍕'); // false, not the same reference
things.get(myFunc); // '😄 Key is a function!'

things.delete('✈️');
things.has('✈️'); // false

things.clear();
things.size; // 0

// setting key-value pairs is chainable
things.set('🔧', 'Wrench')
      .set('🎸', 'Guitar')
      .set('🕹', 'Joystick');

const myMap = new Map();

// Even another map can be a key
things.set(myMap, 'Oh gosh!');
things.size; // 4
things.get(myMap); // 'Oh gosh!'
javascript

迭代它的方法，可以用 for … of，顺序是严格按照你插入的顺序来的：

let activities = new Map();

activities.set(1, '🏂');
activities.set(2, '🏎');
activities.set(3, '🚣');
activities.set(4, '🤾');

for (let [nb, activity] of activities) {
  console.log(`Activity ${nb} is ${activity}`);
}

// Activity 1 is 🏂
// Activity 2 is 🏎
// Activity 3 is 🚣
// Activity 4 is 🤾
fallback

也可以用 forEach 来迭代，注意一点即可，forEach 的 callback 函数，第一个参数是 value，第二个参数是 key，跟 for … of 是相反的：

activities.forEach((value, key) => {
  console.log(`Activity ${key} is ${value}`);
});
fallback

report

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