$(d).attr("id") is undefined javascript

Question

I have a variable d that I use like this:

$(function() {  
    for(i = 1; i <= 31; i++) {
        var d = '#days' + i;    
        if ($(d).attr("id").substr(4,2) == 11) {
            $(d).addClass("date_has_event");
            //console.log("diez");
        } else {
            console.log("otro");
        }
    }
}

However I get the following error in firebug:

$(d).attr("id") is undefined
index.html    (L23)   (?)()()
jquery.min.js (L27)   onreadystatechange()()
jquery.min.js (L27)   onreadystatechange()()
jquery.min.js (L21)   nodeName()([function(), function()], function(), undefined)
onreadystatechange()()

I really don't understand why. Does anyone know?

---

Edit

I'm sorry for the poor explanation I had to run, here's what's going on a little bit more detailed. I am generating a calendar using javascript. each td has a different id (hence the #days + i) and I am running it from 1 to 31 so I can cover the longer months. However I am getting the error I mentioned above. I am also using the jQuery library to enable me to select more easily (i.e. instead of getElementById just #days)

Answer 1

Why not just check if i == 11, then do your processing on it? It would still only fire on $('#days11'). Edit: If you need to make sure the element exists as well, just slap that into the conditional.

$(function(){   
    for(i = 1; i <= 31; i++){
        var d = '#days' + i;    

//       if($(d) && i == 11){            
         if(i == 11){
               $(d).addClass("date_has_event");
               //console.log("diez");
         }else{
               console.log("otro");
         }
    }
}

Answer 2

Ok, new answer. the way you are doing this is not very "jqueryish". lets step back a bit. from what I can tell you have an html structure something like:

<div id="days1"></div>
<div id="days2"></div>
...

You are then running this against every item with a days(num) id? A better solution is this, if you want to add a class to every element with a date in it, first apply a class:

<div class="days"></div>
<div class="days"></div>

Your code can then be

$(function(){
    $(".days").each(function(i){
        if($(this).substr(4,2) == 11){
            $(this).addClass("date_has_event");
        }
    });  
});

Answer 3

Since you are selecting by id, this is redundant:

if ($(d).attr("id").substr(4,2) == 11)

because the ID attribute of d is d.

Is most simple to do:

if (i == 11)

Answer 4

Missing the closing bracket?

$(function() {  
    for(i = 1; i <= 31; i++) {
        var d = '#days' + i;    
        if (i == 11) {
            $(d).addClass("date_has_event");
            //console.log("diez");
        } else {
            console.log("otro");
        }
    }
// shouldn't the next line be });
}