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；
- 唯一键：地图中的每个键都是唯一的。
- 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。
- 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。
