Deconstruction assignment of variables

Deconstruction and assignment of variables

To reduce the definition of variables , And it's easier to extract values from arrays or objects ,ES6 Deconstruction assignment

Deconstruction and assignment of arrays

let[a,b,c]=[1,2,3]

    console.log(a)//1

    console.log(b)//2

    console.log(c)//3

You can omit variables

Can be used with the extension operator

let[a,...b]=[1,2,3,4,5]

            console.log(a)//1

            console.log(b)//[2, 3, 4, 5]

You can define default values

let[a,b,c=0]=[1,2]

            console.log(a)//1

            console.log(b)//2

            console.log(c)//0

Other variables to which the deconstruction assignment can be applied

 let[a,c=a]=[1]

            console.log(a)//1

            console.log(c)//1

Pay attention to the situation

1. The types before and after the deconstruction assignment correspond to

let[a]=null// Report errors

let[a]={}// Report errors

2.undefined Default values cannot be replaced

let[a=0]=[null]//null

let[a=0]=[undefined]//0

3. Reference extension operators and other variables assigned by deconstruction , You can't write backwards

let[...a,b]=[1,2,3,4]// Report errors

let[c=a,a=1]=[]// Report errors

Object and deconstruction assignment

Basic usage

let{name,sex} ={name:" Zhang Liang ",sex:" male "}

           console.log(name)// Zhang Liang

           console.log(sex)// male

Aliasing objects

let{name:a,sex:b} ={name:" Zhang Liang ",sex:" male "}

            console.log(a)// Zhang Liang

            console.log(b)// male

You can define default values

let{name:a,sex:b,age=22} ={name:" Zhang Liang ",sex:" male "}

            console.log(age)//22 

Resolution of nested objects

 var {name,mon:[a,b,c]}={name:" Zhang ",mon:[2000,3000,1000]}

            console.log(name)// Zhang

            console.log(a)//2000

            console.log(b)//3000

            console.log(c)//1000

Deconstruction and assignment of strings

Parse the value of the string

    var [...arr]="hello"

      console.log(arr)//[h,e,l,l,o]

Resolve the properties and methods under the object

            var {length}="hello"

            console.log(length)//5

Parameter deconstruction assignment under function

When parameters are deconstructed for an array

 function fun([a,b,c]){

                console.log(a)

                console.log(b)

                console.log(c)

            }

            fun(1,2,3)

When a parameter is deconstructed for an object

 function fun1({name:a,age:b}){

                console.log(a+","+b)

            }

            fun1({name:"z",age:23})

Be careful ： When the value after the deconstruction assignment is of other types , First consider whether this type can be converted into an object , Otherwise, it cannot be parsed

New attribute method under reference type

Digital extension

0b10101// Define binary

0o767// octal o It can be in both case

Properties and methods

parseInt() Convert to an integer

parseFloat() Convert to floating point number

isNaN() Judge whether it's a number

isFinite() Judge if it's a finite number

isInteger() Is it an integer // Integer returns true

Mathematical expansion

trunc() Remove the decimal part of a number

sign() Judge whether a number is a positive number 、 negative 、 It's still zero

return +1 Positive numbers return -1 negative

return +0 Positive zero return -0 Negative zero

return NaN Other values

cbrt() Calculate the cube root of a number

imul() Returns the product of two numbers , Include a plus or minus sign

hypot() Returns the square root of the sum of squares of parameters

signbit() Judge positive and negative numbers , Returns a Boolean value , Returns a negative number true

console.log(Math.signbit(-5))//true

        console.log(Math.signbit(5))//false

        console.log(Math.signbit(+0))//false

        console.log(Math.signbit(-0))//true

        console.log(Math.signbit("add"))//flase

Index of operation ：2**3// by 8,2 multiply 3 All over =>2*2*2