javascript 中的 map 是键值对的集合，其中键可以是任何类型。它保留插入的顺序，这意味着项目按照添加的顺序进行迭代。

地图的主要特征；

1. 唯一键：地图中的每个键都是唯一的。
2. 键的任何数据类型：与对象不同，键可以是任何数据类型，包括函数、对象或任何原始类型。
3. 可迭代： 您可以迭代 map 的键、值或条目。

基本操作

创建地图

let funnymap = new map();

将元素添加到地图

funnymap.set('a', 1); // string key
funnymap.set(2, 'two'); // number key
funnymap.set(true, 'yes'); // boolean key
funnymap.set({name: 'obj'}, 'object'); // object key

// funny example
funnymap.set('knock-knock', 'who’s there?');

从地图获取值

console.log(funnymap.get('a')); // 1
console.log(funnymap.get(2)); // two
console.log(funnymap.get(true)); // yes

// funny example
console.log(funnymap.get('knock-knock')); // who’s there?

检查密钥

console.log(funnymap.has('a')); // true
console.log(funnymap.has(42)); // false

// funny example
console.log(funnymap.has('chicken')); // false (it crossed the road)

删除元素

funnymap.delete('a');
console.log(funnymap.has('a')); // false

// funny example
funnymap.delete('knock-knock');
console.log(funnymap.get('knock-knock')); // undefined (no one answered)

获取地图的大小

console.log(funnymap.size); // 3 after deletion

// funny example
console.log(`the map has ${funnymap.size} jokes left.`);

清除地图

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

// funny example
console.log(`all jokes are cleared from the map.`);

迭代地图

用于...的

funnymap.set('banana', 'yellow');
funnymap.set('apple', 'red');
funnymap.set('grape', 'purple');

// iterating over keys
for (let key of funnymap.keys()) {
    console.log(`key: ${key}`);
}

// iterating over values
for (let value of funnymap.values()) {
    console.log(`value: ${value}`);
}

// iterating over entries
for (let [key, value] of funnymap.entries()) {
    console.log(`key: ${key}, value: ${value}`);
}

// funny example
funnymap.set('dad joke', 'what do you call fake spaghetti? an impasta!');
for (let [key, value] of funnymap.entries()) {
    console.log(`here’s a ${key}: ${value}`);
}

使用 foreach

funnymap.foreach((value, key) => {
    console.log(`key: ${key}, value: ${value}`);
});

// funny example
funnymap.set('bad pun', 'i’m reading a book on anti-gravity. it’s impossible to put down!');
funnymap.foreach((value, key) => {
    console.log(`here’s a ${key}: ${value}`);
});

实际用途

字数统计（搞笑版）

想象一下您想要计算某些单词在笑话集中出现的次数：

let jokeText = "Why did the scarecrow win an award? Because he was outstanding in his field!";

// Splitting text into words
let words = jokeText.split(/W+/);

let wordCount = new Map();

words.forEach(word => {
    word = word.toLowerCase();
    if (wordCount.has(word)) {
        wordCount.set(word, wordCount.get(word) + 1);
    } else {
        wordCount.set(word, 1);
    }
});

wordCount.forEach((count, word) => {
    console.log(`Word: ${word}, Count: ${count}`);
});

// Funny example
console.log('Word counts in our joke:');
wordCount.forEach((count, word) => {
    console.log(`"${word}": ${count} times`);
});

概括

javascript 中的 map 是一种通用的数据结构，可以保存任何数据类型的键值对，提供简单的迭代方法，并维护元素的顺序。它对于需要比常规对象更灵活的密钥管理的场景非常有用，例如计算笑话中的单词数或存储对各种谜语的响应。

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