Is es6 map ordered?

map is ordered. The map type in ES6 is an ordered list that stores many key-value pairs. The key names and corresponding values ​​support all data types; the equivalence of key names is determined by calling the "Objext.is()" method. Implemented, so the number 5 and the string "5" will be judged as two types, and can appear in the program as two independent keys.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

1. Map collection

The object of JavaScript is essentially a collection of key-value pairs, but traditionally it can only be used as a string. Key-value pairs.

To solve this problem, ES6 provides the map data structure. It is similar to an object and is also a collection of key-value pairs. However, the scope of this key is not limited to strings. Various types of values ​​(including objects) can be used as keys. In other words, the object structure provides (string-value) correspondence, and the map structure implements

The map type in ES6 is an ordered list that stores many key-value pairs, in which the key name and the corresponding Value supports all data types. The equivalence judgment of key names is achieved by calling the Objext.is() method, so the number 5 and the string '5' will be judged as two types and can appear in the program as two independent keys.

Note: There is an exception. 0 and -0 are considered equal in the map collection, which is different from the Object.is() result. If you need "key value" For "data structure, map is more suitable than object, and has extremely fast search speed

1. Attribute: size

Returns map Number of elements

2, basic method

(1) set()

to map Add data and return the added map (assigning a value to an existing key will overwrite the previous value)

(2) get()

Get a certain The value of the key returns the value corresponding to the key. If not, returns undefined

(3) has()

Detects whether a certain key exists and returns a Boolean value

let map = new Map();
 
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');
 
console.log(map.size);
console.log(map.get('JacksonWang'));
console.log(map.get('LEO'));
console.log(map.has('Z-'));
 
//输出:    3
//        123
//        456
//        true

(4)delete()

Delete a key and its corresponding value and return a Boolean value. If the deletion is successful, it will be true

（5）clear()

Clear all values ​​and return undefined

let map = new Map();
 
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');
 
map.delete('Z-');
console.log(map.size);
 
console.log(map.clear())
 
//输出:  2
//      undefined

3. Traversal method

Note: The traversal order of the map is the insertion order

(1) keys()

Get all the keys of the map

（ 2) values()

Get all the values ​​of the map

(3)entries()

Get all the members of the map

let map = new Map();
 
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');
 
console.log(map.keys())//打印所有的键
console.log(map.values())//打印所有的值
console.log(map.entries())//以键值对的方式
 
/*输出：
[Map Iterator] { 'JacksonWang', 'LEO', 'Z-' }
[Map Iterator] { '123', '456', '789' }
[Map Entries] {
  [ 'JacksonWang', '123' ],
  [ 'LEO', '456' ],
  [ 'Z-', '789' ]
}*/

(4) forEach()

Traverse all members of map

let map = new Map();
 
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');
 
for(const [key,value] of map.entries()){
    console.log(`${key}:${value}`);
}
/*输出:
JacksonWang:123
LEO:456
Z-:789
*/

4. Convert to array

Convert map structure to array destructuring

let map1 = new Map([
    [1,'One'],
    [2,'Two'],
    [3,'Three']
])
 
console.log([...map1.keys()]);
console.log(...map1.entries())
console.log([...map1.entries()]);
 
/*输出:
[ 1, 2, 3 ]
[ 1, 'One' ] [ 2, 'Two' ] [ 3, 'Three' ]
[ [ 1, 'One' ], [ 2, 'Two' ], [ 3, 'Three' ] ]
*/

2. Weakmap collection

WeakMap is a weak reference Map collection, also used to store objects Weak reference. The key name in the WeakMap collection must be an object. If you use a non-object key name, an error will be reported: the collection stores weak references to these objects. If there are no other strong references other than the weak references, the engine's garbage collection mechanism This object will be automatically recycled, and the key-value pairs in the WeakMap collection will be removed. But only the key name of the collection follows this rule. If the value corresponding to the key name is an object, the strong reference of the saved object will not trigger the garbage collection device

1, WeakMap collection Purpose

#(1) Store DOM elements

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">WeskMap测试</button>
    <script>
        let btn = document.querySelector(&#39;#btn&#39;);
        let wmap = new WeakMap();
        wmap.set(btn,{timesClicked:0})//在map中放一个键值对，btn为键
 
        btn.addEventListener(&#39;click&#39;,function(){
            let temp = wmap.get(btn);//从这里获取键名为btn的值
            temp.timesClicked++;
            console.log(temp.timesClicked)
        },false)
    </script>
</body>
</html>

myElement in the code is a DOM node, which is updated whenever a click event occurs status. We put this state as a key value in the WeakMap, and the corresponding key name is myElement. Once the DOM node is deleted, the state will automatically disappear, and there is no risk of memory leaks

(2) Register the listener object for listening events

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">WeskMap测试</button>
    <script>
        let btn = document.querySelector(&#39;#btn&#39;);
        let wmap = new WeakMap();
        wmap.set(btn,{timesClicked:0})//在map中放一个键值对，btn为键
 
        // btn.addEventListener(&#39;click&#39;,function(){
        //     let temp = wmap.get(btn);//从这里获取键名为btn的值
        //     temp.timesClicked++;
        //     console.log(temp.timesClicked)
        // },false)
        function f1(){
            let temp = wmap.get(btn);//从这里获取键名为btn的值
            temp.timesClicked++;
            console.log(temp.timesClicked)
        }
        btn.addEventListener(&#39;click&#39;,f1,false)
    </script>
</body>
</html>

The effect is the same

(3) Deploy private properties

function Person(name){
    this._name = name;
}
 
Person.prototype.getName = function(){
    return this._name;
}
//但这是，创建一个Person对象的时候，我们可以直接访问name
let p = new Person('张三');
console.log(p._name)
 
//输出：张三

We don’t want users to directly access the name attribute, so we directly use the following method to wrap name into a private attribute

let Person = (function(){
    let privateData = new WeakMap();
    function Person(yourname){
        privateData.set(this,{_name:yourname})//this当前这个键的对象
    }
    Person.prototype.getName = function(){
        return privateData.get(this)._name;//
    }
    return Person;
})();//定义好了函数就开始执行
 
let p = new Person('jack');
console.log(p._name)//因为name的私有型，所以在外不可访问
console.log(p.getName())
 
/*输出:
undefined
jack
*/

[Related recommendations: javascript video tutorial, webfrontend】

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