Mottie Mottie - 4 months ago 12
jQuery Question

Hidden div height (changing my advice)

Ok, I was going to answer someone's question here on SO about why their script wasn't working. They loaded content into a hidden div, then got the height so they could animate the wrapping div. But I always try to test the code I provide. So I made this demo to prove it to them.

So, umm, have I entered the twilight zone or am I dreaming right now? pinches self OUCH!

I tried that demo in Firefox, IE and Chrome and both methods return the same value. Firebug says zero! I rebooted my computer and I even changed the code a bit (removed the height function) and tried it with jQuery 1.3.2 and it still worked! I know hidden elements USED to return a zero value. Even this SO Answer is giving the advice I would have!

So I guess my question is... did I miss something or are we giving bad advice?

Answer

Looking at the jQuery 1.4.2 and 1.3.2 source code, it actually performs this automatically when you call width() or height() on a hidden element. It sets the following attributes:

{ position: "absolute", visibility: "hidden", display:"block" }

Then it gets the width/height, and restores the old values of the attributes.

So there is no need to change the attributes yourself - jQuery will handle it for you.

<Edit>
This will allow you to get the dimensions of a hidden element. However, it won't work when the element is contained within another hidden element - you'll get a height of 0. In that case you'd need another solution, possibly like this answer.
</Edit>


Here are the relevant bits of the source code from 1.4.2:

cssShow = { position: "absolute", visibility: "hidden", display:"block" },

//[undocumented jQuery.css function which is called by .height() and .width()]
css: function( elem, name, force, extra ) {
    if ( name === "width" || name === "height" ) {
        var val, props = cssShow, ....

        function getWH() {
            ... this function gets the actual width/height into the variable val
        }

        if ( elem.offsetWidth !== 0 ) {
            getWH();
        } else {
            jQuery.swap( elem, props, getWH );
        }

        return Math.max(0, Math.round(val));
    }
    return jQuery.curCSS( elem, name, force );
},

// A method for quickly swapping in/out CSS properties to get correct calculations
swap: function( elem, options, callback ) {
    var old = {};

    // Remember the old values, and insert the new ones
    for ( var name in options ) {
        old[ name ] = elem.style[ name ];
        elem.style[ name ] = options[ name ];
    }

    callback.call( elem );

    // Revert the old values
    for ( var name in options ) {
        elem.style[ name ] = old[ name ];
    }
}
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