前言
类似Java的HashMap与HashSet，TypeScript中也有对应的Map和Set类。
本文主要提供TypeScript中Map和Set类中可以用于刷题的主要方法。
但在开始前，我们先来看下历史上TS/JS中常被用来当做Map用的类 – Object。
1. Object与Map的联系与区别
联系：
- Object is similar to Map that allows you to
- set key-value pairs
- retrieve those values
- delete keys
- detece whether something is stored at a given key
区别：
- Object has a prototype so it has default keys; Map doesn’t.
- The keys of Object are strings, yet there is no type limitation for the keys in Map.
- To get a size of an Object, you have to use something like
Object.keys(obj).length, but for Map you can simply usemap.sizeproperty.
Reference:
https://stackoverflow.com/questions/18541940/map-vs-object-in-javascript
2. 假如用Object当做Map来刷题
1. Shallow copy of obj, spread and Object.assign() 两种方法
const person = {
firstname: 'coding',
lastname: 'bro'
}
// using spread
let p1 = { ...person };
// using Object.assign
let p2 = Object.assign({}, person);
在shallow copy的情况下，假如我做出 p1.firstname = 'haha'， person中的firstname会发生改变吗？
答案是不会。原因是person object中的propery的值是primitive type具体在这个例子里是string类型，所以是被完全复制一份到了p1，然后p1修改它的firstname不会影响到person。
但假如person object中的propery的值是对象类型，那么shallow copy后, person object中的propery是被复制一份到p1, 但这个property的值，也就是那个对象，是由person 和 p1的对应的同名property共享的。 假如，p1对firstnameproperty之下的某个property进行操作，p1的这个操作就会在person中反映出来。
Referecne:
https://developer.mozilla.org/en-US/docs/Glossary/Shallow_copy
2. Deep copy of obj, 假定obj is serializable
let p3 = JSON.parse(JSON.stringify(person));
Reference:
https://developer.mozilla.org/en-US/docs/Glossary/Deep_copy
3. 其他主要的object下使用的方法
/* Set a key-value, get value from a key */
person[firstname] = 'someFirstName';
person[firstname]
Object.entries(person) //[ ['firstname', 'coding'], ['lastname', 'bro'] ]
Object.keys(person) // ['firstname', 'lastname']
Object.values(person) // ['coding', 'bro']
person.hasOwnProperty('firstname') // true
/* Loop through object */
for (let key in person) { } // this for...in loop is not preferred b/c it loops through the properties in the prototype chain as well
for (let key of Object.keys(person)) { }
for (let val of Object.values(person)) { }
for (let entry of Object.entries()) {
console.log(entry[0], entry[1]) // entry[0] - key, entry[1] - value
}
for (let [key, value] of Object.entries(person)) {
console.log(`${key}: ${value}`)
}
3. Map及其方法的若干使用
1. Create a map
let map = new Map<number, number>(); // 设定一个key 与 value类型皆为number的Map
// Create a map with initial values
let map2 = new Map<string, number>([
['key1', 101],
['key2', 102]
]);
2. Add, Retrieve, Delete entries from Map
map.set(key, val) // adds a new entry in the map
map.get(key) // retrives the value for a given key, if the key doesn't exist, return undefined
map.has(key)
map.size
map.delete(key) // deletes a key-value pair using its key. If key's found and deleted, return true; ow. return false
map.clear() // delete all entries from the map
3. Iterating over Map
map.keys() // to iterate over map keys
map.values() // to iterate over map values
map.entries() // to iterate over map entries
map // use object destructuring to iterate over map entries
// 跟iterating through object的写法基本没差 -- 也是4种, 无非就是每一种都用 for...of loop
for (let key of map.keys()) {}
for (let val of map.values()) {}
for (let entry of map.entries()) {
console.log(entry[0], entry[1])
}
for (let [key, value] of map) {
console.log(`${key}: ${value}`)
}
4. Set及其方法的若干使用
1. Create a set
let set = new Set<number>();
// Create a set with initial values
let set2 = new Set<number>([101, 102]);
2. Add, Retrieve, Delete elements from Set
set.add(val) // adds a new value into the set; if the val already exists then it won't be added. Return the set object with added value (or not)
set.has(val) // check if a val exists in the set
set.size
set.delete(val) // deletes the val from the set. Return true if found and deleted; ow. return false
set.clear() // delete all values from the set
3. Iterating over Set
for (let val of set) {}
set.forEach(val => {
// deal with value
})
总结
您还有什么要补充的TypeScript刷题常用的与Map， Set相关的API或者应用吗？如果YES，请留言，不吝赐教。