jquery.pxToEm.js

/*-------------------------------------------------------------------- 
 * javascript method: "pxToEm"
 * by:
   Scott Jehl (scott@filamentgroup.com) 
   Maggie Wachs (maggie@filamentgroup.com)
   http://www.filamentgroup.com
 *
 * Copyright (c) 2008 Filament Group
 * Dual licensed under the MIT (filamentgroup.com/examples/mit-license.txt) and GPL (filamentgroup.com/examples/gpl-license.txt) licenses.
 *
 * Description: Extends the native Number and String objects with pxToEm method. pxToEm converts a pixel value to ems depending on inherited font size.  
 * Article: http://www.filamentgroup.com/lab/retaining_scalable_interfaces_with_pixel_to_em_conversion/
 * Demo: http://www.filamentgroup.com/examples/pxToEm/	 	
 *							
 * Options:  	 								
 		scope: string or jQuery selector for font-size scoping
 		reverse: Boolean, true reverses the conversion to em-px
 * Dependencies: jQuery library						  
 * Usage Example: myPixelValue.pxToEm(); or myPixelValue.pxToEm({'scope':'#navigation', reverse: true});
 *
 * Version: 2.0, 08.01.2008 
 * Changelog:
 *		08.02.2007 initial Version 1.0
 *		08.01.2008 - fixed font-size calculation for IE
--------------------------------------------------------------------*/

Number.prototype.pxToEm = String.prototype.pxToEm = function(settings){
	//set defaults
	settings = jQuery.extend({
		scope: 'body',
		reverse: false
	}, settings);
	
	var pxVal = (this == '') ? 0 : parseFloat(this);
	var scopeVal;
	var getWindowWidth = function(){
		var de = document.documentElement;
		return self.innerWidth || (de && de.clientWidth) || document.body.clientWidth;
	};	
	
	/* When a percentage-based font-size is set on the body, IE returns that percent of the window width as the font-size. 
		For example, if the body font-size is 62.5% and the window width is 1000px, IE will return 625px as the font-size. 	
		When this happens, we calculate the correct body font-size (%) and multiply it by 16 (the standard browser font size) 
		to get an accurate em value. */
				
	if (settings.scope == 'body' && jQuery.browser.msie && (parseFloat(jQuery('body').css('font-size')) / getWindowWidth()).toFixed(1) > 0.0) {
		var calcFontSize = function(){		
			return (parseFloat(jQuery('body').css('font-size'))/getWindowWidth()).toFixed(3) * 16;
		};
		scopeVal = calcFontSize();
	}
	else { scopeVal = parseFloat(jQuery(settings.scope).css("font-size")); };
			
	var result = (settings.reverse == true) ? (pxVal * scopeVal).toFixed(2) + 'px' : (pxVal / scopeVal).toFixed(2) + 'em';
	return result;
};