+1向Jonathan M提供原始方法。由于这是一个明确的货币格式化程序，我继续在输出中添加了货币符号（默认为“$”），并添加了一个默认逗号作为千位分隔符。如果您实际上不需要货币符号（或千位分隔符），只需使用“”（空字符串）作为它的参数。

Number.prototype.formatMoney = function(decPlaces, thouSeparator, decSeparator, currencySymbol) {
    // check the args and supply defaults:
    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces;
    decSeparator = decSeparator == undefined ? "." : decSeparator;
    thouSeparator = thouSeparator == undefined ? "," : thouSeparator;
    currencySymbol = currencySymbol == undefined ? "$" : currencySymbol;

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

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

乔纳森·M的代码对我来说太复杂了，所以我重写了它，在Firefox v30上获得了30%的速度提升，在Chrome v35上获得了60%的速度提升(http://jsperf.com/number-formating2):

Number.prototype.formatNumber = function(decPlaces, thouSeparator, decSeparator) {
    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces;
    decSeparator = decSeparator == undefined ? "." : decSeparator;
    thouSeparator = thouSeparator == undefined ? "," : thouSeparator;

    var n = this.toFixed(decPlaces);
    if (decPlaces) {
        var i = n.substr(0, n.length - (decPlaces + 1));
        var j = decSeparator + n.substr(-decPlaces);
    } else {
        i = n;
        j = '';
    }

    function reverse(str) {
        var sr = '';
        for (var l = str.length - 1; l >= 0; l--) {
            sr += str.charAt(l);
        }
        return sr;
    }

    if (parseInt(i)) {
        i = reverse(reverse(i).replace(/(\d{3})(?=\d)/g, "$1" + thouSeparator));
    }
    return i + j;
};

用法：

var sum = 123456789.5698;
var formatted = '$' + sum.formatNumber(2, ',', '.'); // "$123,456,789.57"

JavaScript中有一个内置函数toFixed：

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

我使用库Globalize（来自Microsoft）：

这是一个很好的项目，可以本地化数字、货币和日期，并根据用户的语言环境以正确的方式自动格式化它们。。。尽管它应该是一个jQuery扩展，但它目前是一个100%独立的库。我建议大家都试试看！：）

已经有好的答案了。这里有一个简单的乐趣尝试：

function currencyFormat(no) {
  var ar = (+no).toFixed(2).split('.');
  return [
      numberFormat(ar[0] | 0),
      '.',
      ar[1]
  ].join('');
}


function numberFormat(no) {
  var str = no + '';
  var ar = [];
  var i  = str.length -1;

  while(i >= 0) {
    ar.push((str[i-2] || '') + (str[i-1] || '') + (str[i] || ''));
    i = i-3;
  }
  return ar.reverse().join(',');
}

然后运行一些示例：

console.log(
  currencyFormat(1),
  currencyFormat(1200),
  currencyFormat(123),
  currencyFormat(9870000),
  currencyFormat(12345),
  currencyFormat(123456.232)
)

Patrick Desjardins（前Daok）的例子对我很有用。如果有人感兴趣，我将其移植到CoffeeScript。

Number.prototype.toMoney = (decimals = 2, decimal_separator = ".", thousands_separator = ",") ->
    n = this
    c = if isNaN(decimals) then 2 else Math.abs decimals
    sign = if n < 0 then "-" else ""
    i = parseInt(n = Math.abs(n).toFixed(c)) + ''
    j = if (j = i.length) > 3 then j % 3 else 0
    x = if j then i.substr(0, j) + thousands_separator else ''
    y = i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + thousands_separator)
    z = if c then decimal_separator + Math.abs(n - i).toFixed(c).slice(2) else ''
    sign + x + y + z