how to check if div has id or not?

Answer 1

if(this.id) is all you need.

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Why will this work?

If the element has an ID, the value will a non-empty string, which always evaluates to true.
If it does not have an ID, the value is an empty string which evaluates to false.

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I am trying to get ids from the loop which are there.

To get a list of IDs, you can use .map like so:

var ids = $(".ui-droppable").map(function() {     
    return this.id ? this.id : null;
}).get();

or use the selector Roko suggests in his answer.