ES6 study notes (five): easily understand the built-in expansion target for ES6

Earlier shared four related ES6 related technologies, such as want to know more, you can check the following connection
"ES6 study notes (a): Easy to get to know object-oriented programming, and object-oriented class"
"ES6 study notes (b): to teach you Fun Object class inheritance and class "
" ES6 study notes (c): teach you to achieve the tab bar CRUD functionality with js object-oriented thinking "
" ES6 study notes (d): teach you to understand the new syntax for ES6 "

Array extension methods

Extended operator (developing syntax)

Extended operator to convert an array or an object as a parameter sequence separated by commas

// 扩展运算符可以将数组拆分成以逗号分隔的参数序列
let arr = [1, 2, 3]
console.log(...arr) // 1 2 3

Extended operator can be applied to merge array

// 扩展运算符应用于数组合并
let arr1 = [1, 2, 3]
let arr2 = [4, 5, 6]
let arr3 = [...arr1, ...arr2]
console.log(arr3) // [1, 2, 3, 4, 5, 6]

// 合并数组的第二种方法
let arr1 = [1, 2, 3]
let arr2 = [4, 5, 6]
arr1.push(...arr2)
console.log(arr1) // [1, 2, 3, 4, 5, 6]

Using the extended operator to traverse the array of classes or objects into the real array

  <div>1</div>
  <div>2</div>
  <div>3</div>

let oDivs = document.getElementsByTagName('div')
console.log(oDivs) //HTMLCollection(3) [div, div, div]
const arr  = [...oDivs];
console.log(arr) //(3) [div, div, div]

Constructor method: Array.from ()

Or converting the array can traverse the object class for the real array

let arrLike = {
	'0': 'a',
  '1': 'b',
  '2': 'c',
  length: 3
}
let arr2 = Array.from(arrayLike) //['a', 'b', 'c']

The method can also accept a second parameter, the array acts like the map method, the elements to be processed, the processed value into the array returned.

let arrLike = {
	'0': 1,
  '1': 2,
  '2': 3,
  length: 3
}
let arr2 = Array.from(arrLike, item => item * 2) 
console.log(arr2) //(3) [2, 4, 6]

Examples of the method: find ()

The first is used to find a qualified member of the array, if not found returns undefined

let arr = [{
            id: 1,
            name: 'lanfeng'
            },
           {
           	id: 2,
             name: 'qiuqiu'
           }
           
          ];
let target = arr.find((item, index) => item.id === 2)
console.log(target) //{id: 2, name: "qiuqiu"}

Examples of the method: findIndex ()

It used to find the first matching array member position, or -1 if not found

let arr = [1, 5, 10, 15]
let index = arr.findIndex(value => value > 9)
console.log(index); //2

Examples of the method: includes ()

It represents an array contains the given value and returns a Boolean value

[1, 2, 3].includes(2) // true
[1, 2, 3].includes(4) // false

String extension method

Template string

ES6 create a string of new ways to define the use of anti pilot number
template string can wrap
the template string can call the function

let name = `zhangsan`
let sayHello = `hello, my name is ${name}`
console.log(sayHello ) // hello, my name is zhangsan

//模板字符串中可以换行
let result = {
	name: 'zhangsan',
  age: 20,
  sex: '男'
}
let html=`<div>
	<span>${result.name}</span>
	<span>${result.age}</span>
	<span>${result.sex}</span>
</div>`;
console.log(html)

// 在模板字符串中是可以调用函数的
const sayHello = function() {
	return 'hello'
}
let greet = `${sayHello()}, lanfeng`
console.log(greet) //hello, lanfeng

Examples of the method: startsWith () and endsWith ()

startsWith (): whether the parameter string representing the head of the string, returns a boolean
endsWith (): Indicates whether the parameter string at the end of the string, returns a Boolean value

let str = 'hello world !'
str.startsWith('hello') // true
str.endsWith('!') //true

Examples of the method: repeat ()

The method represents a repeat n times the original string, returns a new string

const str1 = 'x'.repeat(3)
console.log(str1) // xxx
const str2 = 'hello'.repeat(2)
console.log(str2) // hellohello

Set data structure

ES6 provided Set new data structure. It is similar to the array, but the value of the member are unique, no duplicates
Set itself is a constructor to generate a data structure Set
Set function can accept an array as a parameter, for initialization

const set = new Set([1, 2, 3, 4, 4])
console.log(set.size) // 4 	数组去重
console.log(set) //Set(4) {1, 2, 3, 4}
//转换成数组
console.log([...set]) //[1, 2, 3, 4] 	

Examples of methods

• add (value): add some value, return Set structure itself
• delete (value): delete a value, it returns a Boolean value that indicates whether the deleted successfully
• It has (value): returns a Boolean value that indicates whether the value is
• Set the members of the
  clear (): remove all members, no return value

 const s = new Set();
 s.add(1).add(2).add(3); // 向 set 结构中添加值 
 s.delete(2)             // 删除 set 结构中的2值 
 s.has(1)                // 表示 set 结构中是否有1这个值 返回布尔值 
 s.clear()               // 清除 set 结构中的所有值

Traversal

Examples of Set Structure and arrays, also has forEach method for performing a certain operation on each member, there is no return value

const set = new Set([a, b, c])
set.forEach(item => {
	console.log(item)
})

to sum up

Some examples of methods and usage of these surfaces contain this article is mainly shared objects ES6 built-in expansion of extension methods of Array, String extension method, Set data structure.