Number.extend({

	/*
	Property: numberFormat
		Format a number with grouped thousands.

	Arguments:
		decimals, optional - integer, number of decimal percision; default, 2
		dec_point, optional - string, decimal point notation; default, '.'
		thousands_sep, optional - string, grouped thousands notation; default, ','

	Returns:
		a formatted version of number.

	Example:
		>(36432.556).numberFormat()  // returns 36,432.56
		>(36432.556).numberFormat(2, '.', ',')  // returns 36,432.56
	*/

	numberFormat : function(decimals, dec_point, thousands_sep) {
		decimals = Math.abs(decimals) + 1 ? decimals : 2;
		dec_point = dec_point || '.';
		thousands_sep = thousands_sep || ',';
	
		var matches = /(-)?(\d+)(\.\d+)?/.exec((isNaN(this) ? 0 : this) + ''); // returns matches[1] as sign, matches[2] as numbers and matches[3] as decimals
		var remainder = matches[2].length > 3 ? matches[2].length % 3 : 0;
		return (matches[1] ? matches[1] : '') + (remainder ? matches[2].substr(0, remainder) + thousands_sep : '') + matches[2].substr(remainder).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep) + 
				(decimals ? dec_point + (+matches[3] || 0).toFixed(decimals).substr(2) : '');
	}


});

Number.extend({

	/*
	Property: numberFormat
		Format a number with grouped thousands.

	Arguments:
		decimals, optional - integer, number of decimal percision; default, 2
		dec_point, optional - string, decimal point notation; default, '.'
		thousands_sep, optional - string, grouped thousands notation; default, ','

	Returns:
		a formatted version of number.

	Example:
		>(36432.556).numberFormat()  // returns 36,432.56
		>(36432.556).numberFormat(2, '.', ',')  // returns 36,432.56
	*/

	numberFormat : function(decimals, dec_point, thousands_sep) {
		var decimals = Math.abs(decimals) + 1 ? decimals : 2;
		var dec_point = dec_point || '.';
		var thousands_sep = thousands_sep || ',';
	
		var matches = /(-)?(\d+)(\.\d+)?/.exec(this + ''); // returns matches[1] as sign, matches[2] as numbers and matches[3] as decimals
		var remainder = matches[2].length > 3 ? matches[2].length % 3 : 0;
		return (matches[1] ? matches[1] : '') + (remainder ? matches[2].substr(0, remainder) + thousands_sep : '') + matches[2].substr(remainder).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep) + 
				(decimals ? dec_point + (+matches[3] || 0).toFixed(decimals).substr(2) : '');
	}


});

Number.extend({

	/*
	Property: numberFormat
		Format a number with grouped thousands.

	Arguments:
		decimals, optional - integer, number of decimal percision; default, 2
		dec_point, optional - string, decimal point notation; default, '.'
		thousands_sep, optional - string, grouped thousands notation; default, ','

	Returns:
		a formatted version of number.

	Example:
		>(36432.556).numberFormat()  // returns 36,432.56
		>(36432.556).numberFormat(2, '.', ',')  // returns 36,432.56
	*/

	numberFormat : function(decimals, dec_point, thousands_sep) {
		var decimals = Math.abs(decimals) + 1 ? decimals : 2;
		var dec_point = dec_point || '.';
		var thousands_sep = thousands_sep || ',';
	
		var matches = /(-)?(\d+)(\.\d+)?/.exec(this + ''); // returns matches[1] as sign, matches[2] as numbers and matches[2] as decimals
		var remainder = matches[2].length > 3 ? matches[2].length % 3 : 0;
		return (matches[1] ? matches[1] : '') + (remainder ? matches[2].substr(0, remainder) + thousands_sep : '') + matches[2].substr(remainder).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep) + 
				(decimals ? dec_point + (+matches[3] || 0).toFixed(decimals).substr(2) : '');
	}


});

Number.extend({

	/*
	Property: numberFormat
		Format a number with grouped thousands.

	Arguments:
		decimals, optional - integer, number of decimal percision; default, 2
		dec_point, optional - string, decimal point notation; default, '.'
		thousands_sep, optional - string, grouped thousands notation; default, ','

	Returns:
		a formatted version of number.

	Example:
		>(36432.556).numberFormat()  // returns 36,432.56
		>(36432.556).numberFormat(2, '.', ',')  // returns 36,432.56
	*/

	numberFormat : function(decimals, dec_point, thousands_sep){
		var decimals = Math.abs(decimals) + 1 ? decimals : 2;
		var dec_point = dec_point || '.';
		var thousands_sep = thousands_sep || ',';
	
		var matches = /(\d+)(\.\d+)?/.exec(this + ''); // returns matches[1] to numbers and matches[2] to decimals
		var remainder = matches[1].length > 3 ? matches[1].length % 3 : 0;
		return (remainder ? matches[1].substr(0, remainder) + thousands_sep : '') + matches[1].substr(remainder).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep) + 
				(decimals ? dec_point + (+matches[2] || 0).toFixed(decimals).substr(2) : '');
	};


});

Number.extend({

	/*
	Property: numberFormat
		Format a number with grouped thousands.

	Arguments:
		decimals, optional - integer, number of decimal percision; default, 2
		dec_point, optional - string, decimal point notation; default, '.'
		thousands_sep, optional - string, grouped thousands notation; default, ','

	Returns:
		a formatted version of number.

	Example:
		>(36432.556).numberFormat()  // returns 36,432.56
		>(36432.556).numberFormat(2, '.', ',')  // returns 36,432.56
	*/

	numberFormat = function(decimals, dec_point, thousands_sep){
		var decimals = Math.abs(decimals) + 1 ? decimals : 2;
		var dec_point = dec_point || '.';
		var thousands_sep = thousands_sep || ',';
	
		var matches = /(\d+)(\.\d+)?/.exec(this + ''); // returns matches[1] to numbers and matches[2] to decimals
		var remainder = matches[1].length > 3 ? matches[1].length % 3 : 0;
		return (remainder ? matches[1].substr(0, remainder) + thousands_sep : '') + matches[1].substr(remainder).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep) + 
				(decimals ? dec_point + (+matches[2] || 0).toFixed(decimals).substr(2) : '');
	};


});

Format a number with grouped thousands

Mootools - numberFormat

JavaScript