Mottie Mottie - 1 year ago 61
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 Source

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 ];
    }
}