我想你想要：

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

处理货币输出（包括负数）的函数。样本输出：$5.23-$5.23

function formatCurrency(total) {
    var neg = false;
    if(total < 0) {
        neg = true;
        total = Math.abs(total);
    }
    return (neg ? "-$" : '$') + parseFloat(total, 10).toFixed(2).replace(/(\d)(?=(\d{3})+\.)/g, "$1,").toString();
}

下面是Patrick Desjardins（别名Daok）代码，添加了一些注释和一些小改动：

/*
decimal_sep: character used as decimal separator, it defaults to '.' when omitted
thousands_sep: char used as thousands separator, it defaults to ',' when omitted
*/
Number.prototype.toMoney = function(decimals, decimal_sep, thousands_sep)
{
   var n = this,
   c = isNaN(decimals) ? 2 : Math.abs(decimals), // If decimal is zero we must take it. It means the user does not want to show any decimal
   d = decimal_sep || '.', // If no decimal separator is passed, we use the dot as default decimal separator (we MUST use a decimal separator)

   /*
   According to [https://stackoverflow.com/questions/411352/how-best-to-determine-if-an-argument-is-not-sent-to-the-javascript-function]
   the fastest way to check for not defined parameter is to use typeof value === 'undefined'
   rather than doing value === undefined.
   */
   t = (typeof thousands_sep === 'undefined') ? ',' : thousands_sep, // If you don't want to use a thousands separator you can pass empty string as thousands_sep value

   sign = (n < 0) ? '-' : '',

   // Extracting the absolute value of the integer part of the number and converting to string
   i = parseInt(n = Math.abs(n).toFixed(c)) + '',

   j = ((j = i.length) > 3) ? j % 3 : 0;
   return sign + (j ? i.substr(0, j) + t : '') + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : '');
}

这里有一些测试：

// Some tests (do not forget parenthesis when using negative numbers and number with no decimals)
alert(123456789.67392.toMoney() + '\n' + 123456789.67392.toMoney(3) + '\n' + 123456789.67392.toMoney(0) + '\n' + (123456).toMoney() + '\n' + (123456).toMoney(0) + '\n' + 89.67392.toMoney() + '\n' + (89).toMoney());

// Some tests (do not forget parenthesis when using negative numbers and number with no decimals)
alert((-123456789.67392).toMoney() + '\n' + (-123456789.67392).toMoney(-3));

次要变化包括：

移动了一点Math.abs（小数），只有当不是NaN时才能执行。decimal_sep不能再是空字符串（必须使用某种十进制分隔符）我们使用typeof thousand_sep===“undefined”，如How best to determine if a argument is not send to JavaScript function中所建议的不需要（+n||0），因为这是Number对象

JSFiddle公司

一种仅满足原始要求的极简方法：

function formatMoney(n) {
    return "$ " + (Math.round(n * 100) / 100).toLocaleString();
}

@丹尼尔·马格廖拉：你说得对。以上是一个仓促而不完整的实施。以下是正确的实施方式：

function formatMoney(n) {
    return "$ " + n.toLocaleString().split(".")[0] + "."
        + n.toFixed(2).split(".")[1];
}

这是另一个尝试，只是为了好玩：

function formatDollar(num) {
    var p = num.toFixed(2).split(".");
    return "$" + p[0].split("").reverse().reduce(function(acc, num, i, orig) {
        return num + (num != "-" && i && !(i % 3) ? "," : "") + acc;
    }, "") + "." + p[1];
}

还有一些测试：

formatDollar(45664544.23423) // "$45,664,544.23"
formatDollar(45) // "$45.00"
formatDollar(123) // "$123.00"
formatDollar(7824) // "$7,824.00"
formatDollar(1) // "$1.00"
formatDollar(-1345) // "$-1,345.00
formatDollar(-3) // "$-3.00"

JavaScript中没有“formatNumber”的等价物。你可以自己写，也可以找一个已经这样做的库。