Detect where a font is available. Usage: isFontAvailable("Tahoma") //true

gistfile1.js

var isFontAvailable;
(function(){
	var fontCanvas=null,fontCtx=null,cachedWidth={};
	isFontAvailable=function(fontFace){
		if(!fontCanvas){
			fontCanvas=document.createElement('canvas');
			fontCtx=fontCanvas.getContext('2d');
		}
		var stringToMeasure="0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
		var fontsToTest=["monospace","sans-serif","serif"];
		for(var i=0,len=fontsToTest.length;i<len;i++){
			var thisTestFont=fontsToTest[i];
			var testWidth=cachedWidth[thisTestFont];//cached
			if(!testWidth){
				fontCtx.font="72px "+thisTestFont;
				testWidth=cachedWidth[thisTestFont]=fontCtx.measureText(stringToMeasure).width;
			}

			var finalWidth=cachedWidth[fontFace];
			if(!finalWidth){
				fontCtx.font="72px "+fontFace+","+thisTestFont;
				finalWidth=fontCtx.measureText(stringToMeasure).width;
			}
			if(testWidth!=finalWidth){
				cachedWidth[fontFace]=finalWidth;
				return true;
			}
		}
		return false;
	}
})(this);