1. What is Map
The Map type is an ordered list of key-value pairs, and both keys and values can be of any type
The difference between Map and Set
Set is a data structure called a set, and Map is a data structure called a dictionary
A collection ----- is a combination of a bunch of unordered, associated, and non-repeating memory structures [called elements in mathematics]. A dictionary
----- is a collection of some elements. Each element has a field called key, and the keys of different elements are different
The Set collection stores elements in the form of [value, value], and
the Map dictionary stores elements in the form of [key, value]
2. CRUD
2.1 size
The size attribute returns the total number of members of the Map structure.
const map = new Map();
map.set(‘foo’, true);
map.set(‘bar’, false);
map.size // 2
2.2 set()
sets the key value corresponding to the key name key to value, and then returns the entire Map structure
If the key already has a value, the key value will be updated, otherwise the key will be newly generated
At the same time, the current Map object is returned, which can be written in chain
const m = new Map();
let fn = function(){
}
m.set('edition', 6) // 键是字符串
m.set(fn, 'standard') // 键是函数
m.set(undefined, 'nah') // 键是 undefined
m.set(1, 'a').set(2, 'b').set(3, 'c') // 链式操作
2.3 get()
The get method reads the key value corresponding to the key. If the key cannot be found, it returns undefined
const m = new Map();
const hello = function() {
console.log('hello');};
m.set(hello, 'Hello ES6!') // 键是函数
m.get(hello) // Hello ES6!
2.4 has()
The has method returns a Boolean value indicating whether a key is in the current Map object
const hashMap = new Map();
hashMap.set('edition', 6);
hashMap.set(262, 'standard');
hashMap.set(undefined, 'nah');
hashMap.has('edition') // true
hashMap.has('years') // false
hashMap.has(undefined) // true
2.4 delete()
The delete method deletes a key and returns true. Returns false if deletion failed
const m = new Map();
m.set(undefined, 'nah');
m.has(undefined) // true
m.delete(undefined)
m.has(undefined) // false
2.5 clear()
The clear method clears all members and has no return value
let map = new Map();
map.set('wife', true);
map.set('car', false);
map.size // 2
map.clear()
map.size // 0
3. Traverse
keys()：返回键名的遍历器
values()：返回键值的遍历器
entries()：返回所有成员的遍历器
forEach()：遍历 Map 的所有成员
const map = new Map([
['A', 'no'],
['B', 'yes'],
]);
//keys()
for (let key of map.keys()) {
console.log(key); // "A" "B"
}
//values()
for (let value of map.values()) {
console.log(value); // "no" "yes"
}
// entries()
for (let item of map.entries()) {
console.log(item[0], item[1]);
}
// "A" "no" 、"B" "yes"
// 或者
for (let [key, value] of map.entries()) {
console.log(key, value);
}
// "A" "no" 、"B" "yes"
// 等同于使用map.entries()
for (let [key, value] of map) {
console.log(key, value);
}
// "A" "no" 、 "B" "yes"
map.forEach(function(value, key, map) {
console.log(key, value); // "A" "no" 、 "B" "yes"
});