Set, weakset, map, weakmap in ES6

One 、Set object

1. Definition

set：Set Object allows you to store any type of only value , Whether it's the original value or the object reference . Set Object is a set of values , You can iterate over its elements in the order of insertion . Set The elements in will only appear once , namely Set The elements in are unique （ The definition comes from mdn）set The most important thing in an object is the unique value

2. Usage method

（1）add add value

let set=new Set();
set.add(1);

(2)delete Delete specific values

let set=new Set();
set.add(1);
set.delete(1);

(3)clear Clear all internal values

let set=new Set();
set.add(1);
set.add(5);
set.clear()

(4)has Check whether it contains a value

If there is a value , The return value is true

If it doesn't contain , The return value is false

let set=new Set();
set.add(1);
set.has(1);// The return value is true
set.has(0);// The return value is false

（5）size

Inside size In fact, it is equivalent to our length

 let set = new Set('abcdefg')
 console.log(set.size);

3. Skillfully use Set

It can be used set Achieve de duplication ,set The principle of is that the internal value is unique

 let list=[1,2,3,41,2,1,1]
 let arr=[...new Set(list)]
 console.log(arr);

Two 、WeakSet object

WeakSet： stay WeakSet Is unique in the set of It and Set There are two differences between objects : And Set comparison ,WeakSet It can only be a collection of objects , It can't be any value of any type . WeakSet Hold a weak quote ： A reference to an object in a collection is a weak reference . If there's no other right WeakSet References to objects in , Then these objects will be regarded as The garbage Recycle . It also means that WeakSet There is no list of current objects stored in . Because of this ,WeakSet It can't be enumerated .（ Quote from mdn） In my opinion, the meaning of non enumeration is non iterative cycle ,

In the use of WeakSet When , Be careful not to transfer , The following example is error Of , That is, it cannot be transferred

  let name = { name: "123" }// FALSE , Do not redeposit 
    let ws = new WeakSet([name, { age: 321 }])
   console.log(ws);

The correct method should be directly in WeakSet You can use , Because if it is not used for a period of time, it will be recycled as garbage .

let ws = new WeakSet([{ name: "123" }, { age: 321 }])
  console.log(ws);

3、 ... and 、Map

Before we mentioned set It is inevitable to mention map, First of all, we have a simple understanding of them ,map It is mainly used for query .

Set and Map The main application scenario is Data reorganization and Data storage .

Set It's a data structure called a set ,Map It's a data structure called a dictionary .

  aggregate And Dictionaries The difference between ：  Common ground ： aggregate 、 Dictionaries Can store No repetition Value

                                      Difference ： aggregate In order to [value, value] The form of storage elements , Dictionaries In order to [key, value] In the form of storage

Map The way of use and Set Very similar ,size   clear   has    delete,get     set（ Only these two are different ）set Is the added value ,get Is to get the value

Four 、WeakMap

Turn the first value in the array into an object

 let wm=new WeakMap([
            [{name:"ww"},"xxx"]
        ])
 console.log(wm);