An example of jQuery's problem in determining whether an element exists _ jquery-js tutorial

This article mainly discusses the question about jQuery's determination of whether the element exists. For more information, see this article. Recently, when I was training jQuery, I encountered a problem when jQuery was trying to determine whether the element exists.

The question is as follows: add Id = rad4 after "select button 3". The selected HTML control is followed by the text "select button 4, can be added only once (use js native or JQuery for free)

Function addradio () {if (! Document. getElementById ("rad4") {var main = document. getElementById ("radioContainer"); var input = document. createElement ("input"); input. setAttribute ("type", "radio"); input. setAttribute ("id", "rad4"); var span = document. createElement ("span"); var txt = document. createTextNode ("select button 4"); span. appendChild (txt); main. appendChild (input); main. appendChild (span );}}

It is enough to judge whether an object exists. If (! Document. getElementById ("rad4") But if (! $ ("# Rad4") is a treasure building that is similar to javascript.

In jQuery. Once encapsulated by the $ ("") wrapper, an object is not null or undefined! $ ("# Rad4 ")

Always false. The correct method is as follows:

If no object exists in the package, the length is 0. You only need to do so.

$ (Function () {$ (". domtree p: eq (6) input: eq (1 )"). click (function () {if ($ ("# rad4 "). length <1) {$ ("Select the button 4 "). appendTo ($ (" # radioContainer "));}})})