jquery/src/dimensions.js

// Create innerHeight, innerWidth, outerHeight and outerWidth methods
jQuery.each([ "Height", "Width" ], function( i, name ) {

	var type = name.toLowerCase();

	// innerHeight and innerWidth
	jQuery.fn["inner" + name] = function() {
		return this[0] ?
			jQuery.css( this[0], type, false, "padding" ) :
			null;
	};

	// outerHeight and outerWidth
	jQuery.fn["outer" + name] = function( margin ) {
		return this[0] ?
			jQuery.css( this[0], type, false, margin ? "margin" : "border" ) :
			null;
	};

	jQuery.fn[ type ] = function( size ) {
		// Get window width or height
		var elem = this[0];
		if ( !elem ) {
			return size == null ? null : this;
		}
		
		if ( jQuery.isFunction( size ) ) {
			return this.each(function( i ) {
				var self = jQuery( this );
				self[ type ]( size.call( this, i, self[ type ]() ) );
			});
		}

		return ("scrollTo" in elem && elem.document) ? // does it walk and quack like a window?
			// Everyone else use document.documentElement or document.body depending on Quirks vs Standards mode
			elem.document.compatMode === "CSS1Compat" && elem.document.documentElement[ "client" + name ] ||
			elem.document.body[ "client" + name ] :

			// Get document width or height
			(elem.nodeType === 9) ? // is it a document
				// Either scroll[Width/Height] or offset[Width/Height], whichever is greater
				Math.max(
					elem.documentElement["client" + name],
					elem.body["scroll" + name], elem.documentElement["scroll" + name],
					elem.body["offset" + name], elem.documentElement["offset" + name]
				) :

				// Get or set width or height on the element
				size === undefined ?
					// Get width or height on the element
					jQuery.css( elem, type ) :

					// Set the width or height on the element (default to pixels if value is unitless)
					this.css( type, typeof size === "string" ? size : size + "px" );
	};

});