UPDATE 8/15/2008 - For JavaScript - use

'$' + (parseFloat('123.2')||0).toFixed(2)

instead. I've been using this as of late and forgot to update this post a while ago.

UPDATE 2/18/2008 - Now works with negative values and properly formats '-.006' as '-$0.01' with cent rounding via Math.round() at precision level 2.

A quick function that was put together to parse strings/numbers (float as well) into a US Dollar formatted string. Case in point - we want the number 10000 to actually be turned into a string such as "$10,000.00". This is a common function found in ColdFusion, PHP and the like. The function as follows is not anywhere as robust as it is in other languages - but it's a good start for our entirely USD-based e-commerce applicaitons.

Flash version:

function dollarFormat (origNumber):String {
	var procNum:Number = 0;
	if (typeof origNumber == "string")
		procNum = parseFloat(origNumber);
	else if (typeof origNumber == "number")
		procNum = origNumber;
	else
		return "";
	var isNegative:Boolean = (procNum < 0) ? true : false;
	procNum = Math.abs(procNum);
	var numStr:String = (Math.round(procNum * 100)).toString();
	if (numStr.length == 0 || numStr == "0") {
		return ((isNegative) ? "-" : "") + "$0.00";
	} else if (numStr.length == 1) {
		return ((isNegative) ? "-" : "") + "$0.0" + numStr;
	} else if (numStr.length == 2) {
		return ((isNegative) ? "-" : "") + "$0." + numStr;
	} else {
		var dollarStr:String = numStr.substring(0, (numStr.length - 2));
		var centStr:String = numStr.slice(-2);
		var dollarOutStr:String = "";
		for (var charItr = dollarStr.length; charItr >= 0; charItr--) {
			dollarOutStr = ((((dollarStr.length - charItr) % 3) == 0 && dollarStr.length != charItr && charItr != 0) ? "," : "") + dollarStr.charAt(charItr) + dollarOutStr;
		}
		return ((isNegative) ? "-" : "") + "$" + dollarOutStr + "." + centStr;
	}
}

this can be simply called in an external .as file to be used as an #include or as a static function in an imported actionscript class.

JavaScript version:

function dollarFormat (origNumber) {
	var procNum = 0;
	if (typeof(origNumber) == 'string')
		procNum = parseFloat(origNumber);
	else if (typeof(origNumber) == 'number')
		procNum = origNumber;
	else
		return '';
	var isNegative = (procNum < 0) ? true : false;
	procNum = Math.abs(procNum);
	var numStr = (Math.round(procNum * 100)).toString();
	if (numStr.length == 0 || numStr == '0') {
		return ((isNegative) ? '-' : '') + '$0.00';
	} else if (numStr.length == 1) {
		return ((isNegative) ? '-' : '') + '$0.0' + numStr;
	} else if (numStr.length == 2) {
		return ((isNegative) ? '-' : '') + '$0.' + numStr;
	} else {
		var dollarStr = numStr.substring(0, (numStr.length - 2));
		var centStr = numStr.slice(-2);
		var dollarOutStr = '';
		for (var charItr = dollarStr.length; charItr >= 0; charItr--) {
			dollarOutStr = ((((dollarStr.length - charItr) % 3) == 0 && dollarStr.length != charItr && charItr != 0) ? "," : "") + dollarStr.charAt(charItr) + dollarOutStr;
		}
		return ((isNegative) ? '-' : '') + '$' + dollarOutStr + '.' + centStr;
	}
}

Here's a little test output:

trace(this.dollarFormat(".004"));	// outputs "$0.00"
trace(this.dollarFormat(".005"));	// outputs "$0.01"
trace(this.dollarFormat(".014"));	// outputs "$0.01"
trace(this.dollarFormat(".015"));	// outputs "$0.02"
trace(this.dollarFormat(".019"));	// outputs "$0.02"
trace(this.dollarFormat("000.123"));	// outputs "$0.12"
trace(this.dollarFormat("1.234"));	// outputs "$1.23"
trace(this.dollarFormat("12"));		// outputs "$12.00"
trace(this.dollarFormat("123"));	// outputs "$123.00"
trace(this.dollarFormat("1234"));	// outputs "$1,234.00"
trace(this.dollarFormat("12345"));	// outputs "$12,345.00"
trace(this.dollarFormat("123456"));	// outputs "$123,456.00"
trace(this.dollarFormat("123457"));	// outputs "$123,457.00"
trace(this.dollarFormat("1234578"));	// outputs "$1,234,578.00"
trace(this.dollarFormat("12345789"));	// outputs "$12,345,789.00"