JS obtain elements

1. document.getElementsByTagName ( 'tag name') // Get Returns a pseudo-plurality of array elements by the tag name

<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>

var res=document.getElementsByTagName('div')
console.log(res)

2. document.getElementById ( 'id name') acquired by the element id, returns the corresponding element

<div id="box">哈哈</div>

var res=document.getElementById('box')
console.log(res)

3. By element calls .getElementsByTagName ( 'name tag') 

<div id="box">
  <div>1</div>
  <div>2</div>
</div>
var box=document.getElementById('box')
var res=box.getElementsByTagName('div')
console.log(res)

4. document.getElementsByClassName ( "category name") // Get Returns the name of a plurality of elements by a pseudo-class array

var res=document.getElementsByClassName("box")
console.log(res)

5. By calling .getElementsByClassName elements ( "Class Name")

<div id="app">
<div class="box">1</div>
<div class="box">2</div>
</div>

var app=document.getElementById('app')
var res=app.getElementsByClassName("box")
console.log(res)

6.document.querySelector ( '. Selector') // look through the selector element, a return to the first qualifying

var res=document.querySelector('.box')
console.log(res)

  document.querySelectorAll ( 'selector') // look through the selector element, returns a dummy array

var res=document.querySelectorAll('.box')
console.log(res)