How does jquery determine whether a dom node exists?

How jquery determines whether a dom node exists: 1. Add the exists method to the jquery prototype; 2. Determine whether the length attribute of the current object is greater than 0 in the exists method. If it is greater, it exists; 3. Pass [ $('#id').exist()] just call it.

The judgment idea is as follows:

(Learning video sharing: jquery video tutorial)

1 , first add an exist method on the jquery prototype;

2, then determine whether the length attribute of the current object is greater than 0 in the method, if it is greater than 0, it exists;

3, finally pass $('# Just call id').exist().

Add jquery extension js (directly write a separate js file to store the following code. And introduce it after the jquery code)

(function($) {
 $.fn.exist = function(){ 
  if($(this).length>=1){
   return true;
  }
  return false;
 };
})(jQuery);

Usage method:

The page has the following dom

<div id="mydom">这里是id=dom1节点</div>
<div>这里是DIV节点</div>
<span>这里是span节点</span>

Judgment:

console.log($('#dom').exist()) //返回结果为 false
console.log($('#mydom').exist()) //返回结果为 true
console.log($('div').exist()) //返回结果为 true
console.log($('p').exist()) //返回结果为 false

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