Select element matching ID after a certain element, or create it if it doesn't exist

<div id="foo">
  FOO
</div>

<div id="nextfoo">开发者_如何学C</div>

How can I select #nextfoo from a $('#foo').each()... function, and if it doesn't exist then create it ?

var nextFoo= $('#nextfoo');
if (!nextFoo.length){
    nextFoo=$('<div id="nextfoo"/>').insertAfter('#foo');
}

you mean something like this?

First of all ids should be unique on a given page. So there is no need to have each loop on $("#foo")

You can use jQuery next method to get the next element(#nextfoo). If it is not present then you can insert it using jQuery after method. Try this

if(!$("#foo").next().is('#nextfoo')){
    $("#foo").after('div id="nextfoo" />');
}
else{//It exists
    var nextfoo = $("#foo").next();
}

As others have said, every id should be unique. So if is this structure which you are looking to have repeated...

<div class="foo"></div>
<div class="nextfoo"></div>

... and you are missing some 'nextfoo' classes after your 'foos' then you can do something like the following:

$('.foo').each(function() {
    if (!$(this).next().hasClass('nextfoo')) {
        $(this).after('<div id="nextfoo">nextfoo 2</div>');
    }
});

Here is a demo of this in action.