ES6 0622 III

let

Introduce ： Declare variables

characteristic ：

1、 Declare variables , Variables cannot be declared repeatedly

   let a = 10;
   let a = 20;
   console.log(a); // Identifier 'a' has already been declared

2、 There are block level scopes （ Only valid in the current range ）

{
                let d = 40;
                console.log(d); //40
            }
            // console.log('d', d); //d is not defined

3、 No variable Promotion

console.log(c); //Cannot access 'c' before initialization
let c = 30;

const declare constant

Introduce ： declare constant , Once declared, the value must be assigned

characteristic ：

1、 The value cannot be modified

const A=1;// Reset constant A The assignment will report an error  
A=3;// Uncaught TypeError: Assignment to constant variable.
// error ： Assign to constant 

2、 Must be assigned after declaration

3、 There are block level scopes

 {
                const d = 60;
                console.log(d); //60
            }
            console.log(d);   //d is not defined

Arrow function （ Another article has a detailed explanation ）

CSDNhttps://mp.csdn.net/mp_blog/creation/editor/125264210

Template string  

Introduce ： Enhanced version of string , Wrap in back quotation marks

grammar ：

`${ Variable name }`

explain ：

1、${ Variable name } Called a template placeholder , You can insert any... Here js expression , Arithmetic operators and even functions , You can also embed another template string in the template string

  Example ：

let a = 10
let str = ` Now? ${a} O 'clock `
console.log(str);// Now? 10 O 'clock 

const username = 'alex';
const person = { age: 18, sex: 'male' };
const getSex = function (sex) {
  return sex === 'male' ? ' male ' : ' Woman ';
};

const info = `${username}, ${person.age},${getSex(person.sex)}`;
console.log(info);     //alex, 18, male 

 

Deconstruct assignment

Introduce ： From an array in a certain pattern , Object , And assign values to variables

classification ： Array deconstruction assignment , Object deconstruction assignment

Array deconstruction assignment ：

Introduce ： Extract values from an array in a certain pattern , And assign values to variables

situation ：

situation	Example
When the array length is the same as the number of variables	let [a, b, c] = [1,2,3]; console.log(a,b); // 1 2 3
The length of the array > Number of variables	let [a, b, c] = [1,2,3]; console.log(a,b); // 1 2
The length of the array < Number of variables	let [a, b, c] = [1,2,3]; console.log(a,b,c,d); // 1 2 3 d by undefined

  Object deconstruction assignment

Introduce ： Match the attributes of the object with the name of the variable Match successfully assigned the value of the object property to the variable

let person = { name: 'libai', age: 20 };

let {name, age} = person;
console.log(name1)  // ' libai '
console.log(age1)  //  20

Object deconstructs the alias

let person = { name: 'libai', age: 20 };

let {name: myname , age: myage} = person;
console.log(myname)  // ' libai '
console.log(myage)  //  20