Three use cases of $() in jQuery

This time I will bring you three use cases of jQuery's three $(). What are the precautions when using jQuery's three $(). The following is a practical case, let's take a look.

$ is an alias for jQuery "class", and $() constructs a jQuery object. Therefore, "$()" can be called jQuery's constructor (personal opinion, haha!).

1. $() can be $(expresion), that is, css selector , Xpath or html element, that is, the target element is matched through the above expression.
For example: The object constructed by $("a") uses a CSS selector to construct a jQuery object - it selects all the tags. For example:
$("a").click(function(){...})
is the trigger event when any link on the page is clicked. To be precise, jQuery constructs an object $("a") using the tag , and the function click() is an (event) method of this jQuery object.
For example, there is such a piece of HTML code:

<p>one</p> 
<p> 
<p>two</p> 
</p> 
<p>three</p> 
<a href="#" id="test" onClick="jq()" >jQuery</a>

The operation of this HTML is the following statement:

alert($("p>p").html());

$() is a query expression, also It uses a query expression such as "p>p" to construct a jQuery object, and then "html()" means to display its html content, which is the [two] of the above HTML code snippet. Another example:

$("<p><p>Hello</p></p>").appendTo("body");

$() contains a string. Use such a string to construct a jQuery object, and then add this string to .
2. $() can be $(element), which is a specific DOM element. For example, commonly used DOM objects include document, location, form, etc. Such as this line of code:

$(document).find("p>p").html()); 
$()中的document是一个DOM元素，即在全文寻找带<p>的<p>元素，并显示<p>中的内容。

3. $() can be $(function), that is, a function, which is a shorthand for $(document).ready(). For example, the common form is like this:

$(document).ready(function(){ 
alert("Hello world!"); 
});

Variable operation:

$(function(){ 
alert("Hello world!"); 
});

For selecting elements in HTML documents, jQuery has two methods:
1 ) such as $("p>ul a"), which means the a tag in the ul tag in the p tag
However, there is a difference between $('p>ul') and $('p ul') ,

$(&#39;p>ul&#39;)是<p>的直接后代里找<ul>； 
而$(&#39;p ul&#39;)是在<p>的所有后代里找<ul>。

2) Use several methods of the jQuery object (such as methods find(), each(), etc.)
$("#orderedlist).find("li") Just Like $("#orderedlist li").each() iterates all li, and the "#" in the expression represents the ID in HTML. For example, "#orderedlist" in the above example means "the ID is where orderedlist is located" Label".
******************************************** **********************
1, tag selector$('p'), class selector $('.myClass') and id selector $('#myId') are relatively simple, not much to say, but there is one difference - there is a difference between $('p>ul') and $('p ul'). ,
$('p>ul') is to find

among the direct descendants of

; and $('p ul') is to find

among all the descendants of

.
So, $('#sId>li') selects all

• child nodes with the id "sId", even if the descendants of this
• still have
• The range found (the DOM object found is only the DOM object of its own level.). And $('#sId li:not(.horizontal)') refers to all the descendants of li in the class name "sId". There are no elements of the horizontal class. ——not() here is a negation pseudo class.
  What is returned here is a jQurey object, an array object, and the length of this jQuery object can be obtained with .length()
  . #2. attribute; it is a
  attribute selector
  There is no @ in [], indicating that [] is the descendant of the element $('ul li') and $('ul[li]. ')Although both return a jQuery array, the meanings of the two are exactly the opposite. The former is to find all the descendants of
• under
  , while the latter is to find the
  array whose descendants are
  • .
    In XPath, if you want to find an attribute "starting with...", use ^=. If you want to find an input element whose name attribute starts with mail, use
    $('input[@name^ ="mail"]')
    To find an attribute "ending with...", use $=
    To find an attribute "without beginning or end", use *=
    3, Selectors that do not belong to the above-mentioned CSS and XPath are custom selectors, represented by ":". What is used here is:
    
    

    first，:last，:parent ，:hidden，:visible，:odd，:even，:not('xxx')， ":eq(0)"（始于0)，:nth(n），:gt(0)，:lt(0)，:contains("xxx")

    如：$('tr:not([th]):even')意为元素的子孙中不含的所有子孙的偶数项
    4、还有几个，简单不解释了
    

    $('th').parent()—— 
    $(&#39;td:contains("Henry")&#39;).prev()——内容包含有"Henry"的<td>的上一个节点 
    $(&#39;td:contains("Henry")&#39;).next()——内容包含有"Henry"的<td>的下一个节点 
    $(&#39;td:contains("Henry")&#39;).siblings()——内容包含有"Henry"的<td>的所有兄弟节点

    还有一个，就是end()，这个方法肯定是用在某个DOM节点执行了某一动作之后，还想在与其相关的节点上执行类似动作，这里就要用到end()。用过end()方法之后，所返回的是执行动作的那个节点的父节点上。举个例子
    

    $(...).parent().find(...).addClass().end()

    这里执行动作的节点是find(...)，是一个数组对象，它所做的动作是“addClass()”，之后，用了个end()，这时所返回的东东就是指向了parent()所指向的节点，也就是执行“addClass()”动作的那个数组对象的父节点。
    5、要直接访问DOM元素，可用get(0)的方法，如
    

    $('#myelement').get(0)，也可缩写成$('#myelement')[0]

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