我想你想要：

f.nettotal.value = "$" + showValue.toFixed(2);

PHP函数“number_format”有一个JavaScript端口。

我发现它非常有用，因为它易于使用，并且对PHP开发人员来说是可识别的。

function number_format (number, decimals, dec_point, thousands_sep) {
    var n = number, prec = decimals;

    var toFixedFix = function (n,prec) {
        var k = Math.pow(10,prec);
        return (Math.round(n*k)/k).toString();
    };

    n = !isFinite(+n) ? 0 : +n;
    prec = !isFinite(+prec) ? 0 : Math.abs(prec);
    var sep = (typeof thousands_sep === 'undefined') ? ',' : thousands_sep;
    var dec = (typeof dec_point === 'undefined') ? '.' : dec_point;

    var s = (prec > 0) ? toFixedFix(n, prec) : toFixedFix(Math.round(n), prec);
    // Fix for Internet Explorer parseFloat(0.55).toFixed(0) = 0;

    var abs = toFixedFix(Math.abs(n), prec);
    var _, i;

    if (abs >= 1000) {
        _ = abs.split(/\D/);
        i = _[0].length % 3 || 3;

        _[0] = s.slice(0,i + (n < 0)) +
               _[0].slice(i).replace(/(\d{3})/g, sep+'$1');
        s = _.join(dec);
    } else {
        s = s.replace('.', dec);
    }

    var decPos = s.indexOf(dec);
    if (prec >= 1 && decPos !== -1 && (s.length-decPos-1) < prec) {
        s += new Array(prec-(s.length-decPos-1)).join(0)+'0';
    }
    else if (prec >= 1 && decPos === -1) {
        s += dec+new Array(prec).join(0)+'0';
    }
    return s;
}

（原文注释栏，包括以下示例和到期信用）

// Formats a number with grouped thousands
//
// version: 906.1806
// discuss at: http://phpjs.org/functions/number_format
// +   original by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
// +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
// +     bugfix by: Michael White (http://getsprink.com)
// +     bugfix by: Benjamin Lupton
// +     bugfix by: Allan Jensen (http://www.winternet.no)
// +    revised by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
// +     bugfix by: Howard Yeend
// +    revised by: Luke Smith (http://lucassmith.name)
// +     bugfix by: Diogo Resende
// +     bugfix by: Rival
// +     input by: Kheang Hok Chin (http://www.distantia.ca/)
// +     improved by: davook
// +     improved by: Brett Zamir (http://brett-zamir.me)
// +     input by: Jay Klehr
// +     improved by: Brett Zamir (http://brett-zamir.me)
// +     input by: Amir Habibi (http://www.residence-mixte.com/)
// +     bugfix by: Brett Zamir (http://brett-zamir.me)
// *     example 1: number_format(1234.56);
// *     returns 1: '1,235'
// *     example 2: number_format(1234.56, 2, ',', ' ');
// *     returns 2: '1 234,56'
// *     example 3: number_format(1234.5678, 2, '.', '');
// *     returns 3: '1234.57'
// *     example 4: number_format(67, 2, ',', '.');
// *     returns 4: '67,00'
// *     example 5: number_format(1000);
// *     returns 5: '1,000'
// *     example 6: number_format(67.311, 2);
// *     returns 6: '67.31'
// *     example 7: number_format(1000.55, 1);
// *     returns 7: '1,000.6'
// *     example 8: number_format(67000, 5, ',', '.');
// *     returns 8: '67.000,00000'
// *     example 9: number_format(0.9, 0);
// *     returns 9: '1'
// *     example 10: number_format('1.20', 2);
// *     returns 10: '1.20'
// *     example 11: number_format('1.20', 4);
// *     returns 11: '1.2000'
// *     example 12: number_format('1.2000', 3);
// *     returns 12: '1.200'

一个简单的选项，通过先反转字符串和基本正则表达式来正确放置逗号。

String.prototype.reverse = function() {
    return this.split('').reverse().join('');
};

Number.prototype.toCurrency = function( round_decimal /*boolean*/ ) {       
     // format decimal or round to nearest integer
     var n = this.toFixed( round_decimal ? 0 : 2 );

     // convert to a string, add commas every 3 digits from left to right 
     // by reversing string
     return (n + '').reverse().replace( /(\d{3})(?=\d)/g, '$1,' ).reverse();
};

PatrickDesjardins的回答看起来不错，但我更喜欢简单的JavaScript代码。下面是我刚刚编写的一个函数，用于输入一个数字并以货币格式返回（减去美元符号）：

// Format numbers to two decimals with commas
function formatDollar(num) {
    var p = num.toFixed(2).split(".");
    var chars = p[0].split("").reverse();
    var newstr = '';
    var count = 0;
    for (x in chars) {
        count++;
        if(count%3 == 1 && count != 1) {
            newstr = chars[x] + ',' + newstr;
        } else {
            newstr = chars[x] + newstr;
        }
    }
    return newstr + "." + p[1];
}

JavaScript中有一个内置函数toFixed：

var num = new Number(349);
document.write("$" + num.toFixed(2));

这里是XMLilley提供的代码的mootools1.2实现。。。

Number.implement('format', function(decPlaces, thouSeparator, decSeparator){
decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces;
decSeparator = decSeparator === undefined ? '.' : decSeparator;
thouSeparator = thouSeparator === undefined ? ',' : thouSeparator;

var num = this,
    sign = num < 0 ? '-' : '',
    i = parseInt(num = Math.abs(+num || 0).toFixed(decPlaces)) + '',
    j = (j = i.length) > 3 ? j % 3 : 0;

return sign + (j ? i.substr(0, j) + thouSeparator : '') + i.substr(j).replace(/(\d{3})(?=\d)/g, '$1' + thouSeparator) + (decPlaces ? decSeparator + Math.abs(num - i).toFixed(decPlaces).slice(2) : '');
});