ES6 introduction:
ES6 is a set of standards, javascript element is the perfect realization of this set of standards. Follow this set of standard ES6 syntax
1. Let declare variables
In vscode
<script>
// var 声明的变量没有局部作用域
// let 声明的变量 有局部作用域
//创建代码块
{
var a = 0
let b = 1
}
console.log(a) // 0
console.log(b) // ReferenceError: b is not defined
</script>
This is the output from the console. The default output below is in the comments below
// var 可以声明多次
// let 只能声明一次
var m = 1
var m = 2
let n = 3
let n = 4
console.log(m) // 2
console.log(n) // Identifier 'n' has already been declared
2, const declaration constant (read-only variable)
Create const.html
// 1、声明之后不允许改变
const PI = "3.1415926"
PI = 3 // TypeError: Assignment to constant variable.
// 2、一但声明必须初始化，否则会报错
const MY_AGE // SyntaxError: Missing initializer in const declaration
3. Deconstruction and assignment
Create destructuring assignment.html
Destructuring assignment is an extension of the assignment operator.
It is a pattern matching for arrays or objects, and then assigning values to the variables.
The code writing is concise and easy to read, and the semantics is clearer; it also facilitates the acquisition of data fields in complex objects.
//1、数组解构
// 传统
let a = 1, b = 2, c = 3
console.log(a, b, c)
// ES6
let [x, y, z] = [1, 2, 3]
console.log(x, y, z)
//2、对象解构
let user = {name: 'Helen', age: 18}
// 传统
let name1 = user.name
let age1 = user.age
console.log(name1, age1)
// ES6
let { name, age } = user//注意：结构的变量必须是user中的属性
console.log(name, age)
4. Template string
Create template string.html
The template string is equivalent to the enhanced version of the string, with the backquote `, in addition to being a normal string, it can also be used to define a multi-line string, and you can also add variables and expressions to the string.
// 1、多行字符串
let string1 = `Hey,
can you stop angry now?`
console.log(string1)
// Hey,
// can you stop angry now?
// 2、字符串插入变量和表达式。变量名写在 ${} 中，${} 中可以放入 JavaScript 表达式。
let name = "Mike"
let age = 27
let info = `My Name is ${name},I am ${age+1} years old next year.`
console.log(info)
// My Name is Mike,I am 28 years old next year.
// 3、字符串中调用函数
function f(){
return "have fun!"
}
let string2 = `Game start,${f()}`
console.log(string2); // Game start,have fun!
5. Shorthand for declaring objects
Shorthand for creating declaration object.html
const age = 12
const name = "Amy"
// 传统
const person1 = {age: age, name: name}
console.log(person1)
// ES6
const person2 = {age, name}
console.log(person2) //{age: 12, name: "Amy"}
6. Abbreviation of definition method
Create definition method abbreviation.html
// 传统
const person1 = {
sayHi:function(){
console.log("Hi")
}
}
person1.sayHi();//"Hi"
// ES6
const person2 = {
sayHi(){
console.log("Hi")
}
}
person2.sayHi() //"Hi"
7. Object expansion operator
Create object expansion operator. html
expansion operator (...) is used to take out all the traversable attributes of the parameter object and copy it to the current object.
// 1、拷贝对象
let person1 = {name: "Amy", age: 15}
let someone = { ...person1 }
console.log(someone) //{name: "Amy", age: 15}
// 2、合并对象
let age = {age: 15}
let name = {name: "Amy"}
let person2 = {...age, ...name}
console.log(person2) //{age: 15, name: "Amy"}
8. Arrow function
Create arrow functions.html
Arrow functions provide a more concise way of writing functions. The basic syntax is:
parameter => function body
// 传统
var f1 = function(a){
return a
}
console.log(f1(1))
// ES6
var f2 = a => a
console.log(f2(1))
// 当箭头函数没有参数或者有多个参数，要用 () 括起来。
// 当箭头函数函数体有多行语句，用 {} 包裹起来，表示代码块，
// 当只有一行语句，并且需要返回结果时，可以省略 {} , 结果会自动返回。
var f3 = (a,b) => {
let result = a+b
return result
}
console.log(f3(6,2)) // 8
// 前面代码相当于：
var f4 = (a,b) => a+b
Arrow functions are mostly used in the definition of anonymous functions