jquery: what does “$(this)” exactly mean?

I have a program and it works well. See HERE.

This is the code:

<div id="round"></div>

<style>
#round{
position: absolute;
width: 200px;
height: 200px;
border-radius: 50%;
left: 400px;
top: 200px;
background-color: #e1e1e1;
}
</style>

<script src="jquery.js"></script>
<script src="jquery.easing.1.3.js"></script>
<script>
$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $("#round").animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});
</script>

But when I change "#round" to "this". It won't work. why? (actually it works, but when I put them into setInterval(), it won't work)

$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $("#round").animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});

change to "this", it won't work.

$(document).ready(function(){
    $("#round").click(function(){
        setInterval(function(){
            $(this).animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});

Answers:

Answer

Inside of a jQuery bound event function, this refers to the current DOM element from the collection which is being operated on. Since it is a DOM element, passing it to jQ like $( this ) makes it a jQuery collection so that you can do more jQuery stuff to it.

In your modified, non-workng code, however, you moved that into a new anonymous function. Inside of that function, this now refers to the new scope.

You need to get a reference to this before your function:

$(document).ready(function(){
    $("#round").click(function(){
        var jQuerizedElement = $( this );
        setInterval(function(){
            jQuerizedElement.animate(
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});
Answer

$(this) is context sensitive. Each [anonymous, in this case] function you're entering, the value of $(this) changes. For example:

$('#round').click(function(){
    alert($(this).attr('id')) // alerts round
    setInterval(function(){
        alert($(this).attr('id')) // alerts undefined
    });
});
Answer

this is basically context related. When you say $(this) if this is a dom element it will give you jquery object associated with this dom element.

Answer

Because you are using a callback function that is fired by setInterval on a different context...

You can handle this by copying 'this' to an other variable ex :

var that = this:

And on callback

$(that).animate...
Answer

If I understand well it seems that $(this) is just the node that was triggerd.

For instance when you have a click event on a button. In the callback of the event you can use $(this) that represent the node of the button...

Answer

this is a reference to the member that invokes the current function...

then you can wrap it in the jquery function $() to select it just like you would another selector.

So setInterval calls a anonymous function so it is not invoked by a referencable member, so it defaults to the window object.

save the this context in a variable and then use it internally like this...

$(document).ready(function(){
    $("#round").click(function(){
        var clicked = this;   //<----store the click context outside setInterval
        setInterval(function(){
            $(clicked).animate(  //<----------use it here
                {height: 250,
                width: 150,
                top:150,
                left: 425},
                {duration: 300}
                ).
            animate(
                {height: 200,
                width: 200,
                top:200,
                left: 400},
                {duration: 300}
            );
        }, 0);
    });
});

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