JSCORE day_ 04(7.5)

review

• yi Precise configuration of objects :

  • Advanced knowledge points : You can accurately configure the properties of objects

  • configurable: Reconfigurable

  • writable: New values can be written

  • enumerable: Ergodic -- for...in

  • value: value

  • get: Compute properties . The value of the property is a function , You don't need () It will automatically trigger

  • set: Monitor . Property becomes a function , Receive the value passed in by the assignment operation , To test

    • Satisfy : Storage Attributes made for storing data , The name is usually _???

    • dissatisfaction : Report errors

• How to protect objects

  • Prevent increase : preventExtensions

  • Prevent additions and deletions : seal

  • Prevent addition, deletion and modification : freeze

• Arrow function : More convenient anonymous function format

  • Grammatical sugar :

    • There is only one formal parameter , () Omit

    • The function body has only one line , It can be omitted return and {}

  • this Point to :

    • No, this, Use the upper scope this-- Prototype chain mechanism

• Template string

  • ES6 New string enhanced syntax

  • Optimized string splicing : ${}

  • Support line break : The main response is HTML Multiple lines of code

• Array higher order functions : Higher order - Other functions are used in the function

  • every: Everyone is satisfied

  • some: At least one satisfies

  • filter: Filter out those that meet the conditions

  • map: mapping -- The return value after processing the array elements , Make a new array

One 、 Higher order function forEach

About traversing arrays , Now there are at least four

  Traditional scheme : for loop

 for...in: General scheme of traversing object types ; Array is a special object

  ES6: Provides for..of Specifically for traversing arrays , And for..in The difference is that it directly traverses the value , There is no serial number

  ES6: Provide an array of Higher order function forEach, Be careful : Different from other higher-order functions , It has no return value , Just traverse

  application forEach A case of

  Two 、reduce

The traditional way

 reduce The way

 reduce: Array element merging , Pay attention to the meaning of parameters

  3、 ... and 、let And const

 

 ES6 Before , Declared variables are only var and function, There is a global pollution risk : Directly declare in the script that it will appear in window In the object ;ES6 Provided in the let and const key word .

1. Declared variables are stored in Script scope ( New scope ), The script scope must be viewed with the breakpoint technique .

 2. let Declare changeable ;const declare constant : It cannot be changed after the declaration - More secure , More efficient .

    Case study : Choose what statement to use according to the situation ?
    Daming's salary salary  -- The prediction may change in the later stage ; Daming's wife wife  -- Expect no change in the future  

Theoretical priority :  const > let   no need var

 3. About declaration promotion （ Interview questions . a key ）
const and let There are statements to improve , But exist “ Temporary deadband ” Set up . First , Declare that promotion is not a good design , So the author has to make changes ; however , The declaration of ascension is deeply rooted , Directly modifying its source code has too much influence ; So the author provides an error report , You have to declare it before you use it , Otherwise, the backstage will be popular .
let/const Variable , Before declaring code execution , be called Temporary deadband state -- Temporary promotion , however A dead state , Not allowed to use .   Can only be used after declaring code execution

   

  Four 、 Block level

Block level scope : Used instead of closures

Closure : Create a function scope with anonymous functions , Declare variables in it For other functions

Block level : use {} coordination let/const Use

 ES6 Before that, we used closures

  Some grammars come with {}, Can form a block level , To avoid global pollution for  if while switch All have their own block level
    myth : function {}   Function scope
    object {}:   Although there are {} But I didn't write it let The grammar of

  5、 ... and 、 Expand grammar ：...

Merge two arrays using  “ ... ” Remove the outer layer of the array [ ]

  Application scenarios , For example, in the use of max Function time , It only accepts Pass in numbers one by one as parameters , Do not accept arrays , Find the maximum value in an array , Just use the expansion grammar .

  alike , Objects can also use expansion syntax , But it encounters an attribute with the same name , What you write later will cover what you write first .

6、 ... and 、 deconstruction

Optional deconstruction : Just the first and the last one

When we store the elements in the array in a b c d Among the four variables , Using deconstruction syntax is much more convenient , You don't have to write a code for each element .

Flexible application scenarios : Values of interchangeable variables , Originally, we need to define another variable to store data , But now it's not needed . But here's the thing , After declaring variables, you need to use semicolons to separate the code that deconstructs the syntax , Prevent the value assigned to the variable in front of the printout .

When we need to read out the attributes in the object , Deconstruct two properties , You can also use alias syntax : Deconstruct the attributes Change your name , Format ---- “ Property name : Alias ”.

  In actual combat : The requested data is usually object Doped array Structure , It can also be deconstructed .

  Array Can also deconstruct

Parameters of deconstruction

 

js turn html+ Parameters of deconstruction

  7、 ... and 、 Parameter default

 ES6 Function enhanced syntax : Parameter default . When no arguments are passed , The default value of the formal parameter ; When passing arguments , Then use the passed in value Instead of default .

  8、 ... and 、 The remaining parameters of the function

  The remaining parameters :   Than arguments More powerful And flexibility

  arguments: Receive and store all arguments
    shortcoming 1: Implicit ,   Some programmers don't know arguments The existence of
    shortcoming 2: Pseudo array type , There are no array related methods

  ES6 New syntax :   use  ... Variable name   Explicitly declare parameters , Receive all arguments
      this ... and Previous expansion ... It doesn't matter , It's just the same in appearance