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ES6 new feature (1): let command / const command / deconstruction assignment / symbol / set / weakset

2022-04-21 18:20:00 【Hua Weiyun】

（ One ）let command

1.let The declared variable is valid only in the code block in which the variable is declared

contrast ：

{var url = "www.baidu.com";}alert(url);//--------------------------------{let url = "www.baidu.com";}alert(url);

2.let It is not allowed to declare variables repeatedly in the same scope :

let url = "www.baidu.com";{  let url="www.sina.com.cn";  console.log(url);}

correct , Error in parentheses

3.let Declaration of global variables

// error

let a=3;  {    var a=5;  }  alert(a);

// correct

var a=3;{   let a=5;}alert(a);

4. Scope to child scope

let a=3;    {       a=5;  // Subscope     }    alert(a); // eject 5

（ Two ） const command

const Command is used to generate a constant , After constant is declared , The value cannot be changed . Others and let identical .

（ 3、 ... and ） Deconstruct assignment

1. An array of deconstruction

let [a, b, c,d] = ["aa", "bb", 77,88];

Nested array deconstruction

let [a,b,[c,d],e] =["aa",'bb',[33,44],55];

Vacancy variable

let [a,b,,e] =["aa",'bb',[33,44],55];

Redundant variables

let [a,b,,e,f] =["aa",'bb',[33,44],55];

The default value is

let [a,b,,e,f='hello'] =["aa",'bb',[33,44],55];

2. Object to deconstruct

let obj={uid:121,uname:' Zhang San '};let obj=new Object();obj.uid=111;obj.uname=' Zhang San ';let {uid:id,uname:name}=obj; // Sequence change has no effect alert(name); parentheses ： let uid,uname;({uid,uname} = obj); // Must have parentheses , otherwise {} It will be interpreted as a statement block alert(uname);

Can be nested

There can be default values

3. String deconstruction

let [a, b, c, d] =" Rely on the sky to kill the Dragon ";console.log(a,b,c,d);

4. Deconstruction of function parameters

let obj={uid:121,uname:' Zhang San '};function analysis({uid,uname}){    alert(uid);    alert(uname);}analysis(obj);//------- The following is also true function analysis({uname}){  alert(uname);}

Array in parameter 、 character string 、 The default value is 、 All vacancies support

（ Four ）Symbol

Symbol yes ES6 A new value type data , Represents a value that never repeats

let s1 = Symbol(33);let s2 = Symbol(33);alert(s1.toString()); // Symbol(33);alert(s1==s2);    // Report errors

（ 5、 ... and ）Set

Set and WeakSet The data structure is ES6 newly added .

It is very similar to arrays , however Set The members of the data structure are unique . Element cannot be duplicated .

In particular :Set Only one... Can be added to the NaN.

Set data structure :

var set = new Set([1, 2, 3, 4, 2, 8, 4]); // Two 2   //set Traverse for (var elem of set) {     console.log(elem)   }   //----------- Recycle add Add assignment -----------------------   var set = new Set();   [1, 2, 3, 4, 2, 8, 4].map(function (elem) {     set.add(elem);   })   for (let elem of set) { //for...of Traverse      console.log(elem)   }   //----------- Extension operator （…）--------------   var set = new Set([1, 2, 3, 4, 2, 8, 4]);   var arr = [...set];// Extension operator （…） For internal use for…of loop ,   console.log(arr);   //12348   //------ Empty 、 Delete ----------   var set = new Set([1, 2, 3, 4, 2, 8, 4]);   set.clear(); // Empty    set.delete(8); // Delete    var arr = [...set];    console.log(arr);      //------ Traverse the key values ------------   set.forEach(function (value, key) {       console.log(value+'='+key);   })

As can be seen from the output , The key and key value are the same .

//----- Judge set Whether contains ----   console.log(set.has(8));   //---- Traversal value ( Chicken ribs )--------   let setIter = set.values();     for(let val of setIter) {       console.log(val);     }   //---- Number ---------   console.log(set.size);   //-------map and filter It can also be used for set in --------

es5 New array in map and filter Method

map: The meaning of mapping , The mapping returns a new array , There is a return value ;filterArr: Return a new object

filter: Filter 、 Screening means ; There is a return value ; filterArr: Returns the filtered new array

var filterArr = array.filter(function(value, index, array ) {  if ( value > 20 ) {    return true;  }  return false;});

//---------------------------------------------   let set = new Set([1, 2, 3]);   set = new Set([...set].map(x => x * 2));   var arr = [...set];   console.log(arr);   //  return Set structure ：{2, 4, 6}      let set = new Set([1, 2, 3, 4, 5]);   set = new Set([...set].filter(x => (x % 2) == 0));   var arr = [...set];   console.log(arr);   //  return Set structure ：{2, 4}   //--------------- Union 、 intersection 、 Difference set --------------     // Therefore use Set You can easily implement Union （Union）、 intersection （Intersect） And subtraction （Difference）.

（ 6、 ... and ）WeakSet

It is associated with Set Very similar , The value of an object cannot be duplicate , And Set Difference :

1.WeakSet Members can only be objects .

2. As WeakSet Members are all weakly referenced objects , That is, garbage collection mechanism does not consider WeakSet Reference to the object , in other words , If no other object references that object , Then the garbage collection mechanism will automatically reclaim the memory occupied by the object , Regardless of whether the object still exists in WeakSet In . What does this feature mean , Can't quote WeakSet Members of , therefore WeakSet It's not ergodic .

3. Use WeakSet The benefits of storing object instances are , Because it is a reference to an object instance , Will not be included in the memory recycling mechanism , So when deleting an instance , Don't have to consider weakset, There will be no memory leaks .

//--- error ---   var weakset = new WeakSet();   weakset.add(5);   //--- correct ---   var weakset = new WeakSet();   weakset.add({});        // add to    var weakset = new WeakSet([[1, 2], [3, 4]]);   //--- Example ------------------------------------   var weakset = new WeakSet();     let aObj = {a:'aa'};     let bObj = new String(" Hello ");     let cObj = new Number(8);     weakset.add(aObj);     weakset.add(bObj);     weakset.add(cObj);   //--- Delete -----   weakset.delete(aObj);   bObj=null;      // Delete the object ,weakset The object in is gone    console.log(weakset.has(bObj)); //【 purpose ：】weakset Cannot take value , It doesn't show , Only used to indicate whether there are duplicate objects