Javascript返回NaN [英] Javascript returning NaN

问题描述

每次客户输入数量时，我都会尝试显示小计。但是，当我循环输入时，我得到一个 NaN 作为总数。我相信这可能是我在我的脚本中声明小计变量的方式，但我之前没有遇到过这个：

I'm attempting to display a subtotal each time a customer enters a quantity. However, when I loop through my inputs, I get a NaN as the total. I believe it may be the way I'm declaring the subtotal variable in my script, but I haven't come across this before:

$('.item-qty input').bind('keyup', function(){

   var subtotal = 0.0;

   $('.item-qty input').each(function(){
      var class = $(this).attr('id');
      var qty = $(this).val();                     
      var price = $('.'+class).html();

      price = parseFloat(price);
      qty = parseInt(qty);
      subtotal = subtotal + (price * qty);
   });

   $('.subtotal input').val(subtotal);
});

推荐答案

parseFloat 和 parseInt 如果第一个字符可以返回 NaN 该字符串无法转换为数字。

parseFloat and parseInt can return NaN if the first character of the string cannot be converted to a number.

因此，我会像这样保护它（ NaN 是a falsy 值）：

So, I would safeguard against it like this (NaN is a falsy value):

price = parseFloat(price) || 0;
qty = parseInt(qty, 10) || 0;

对数字进行算术运算，其值 NaN 几乎总是导致 NaN （ NaN + 5 将导致 NaN 。）这意味着，如果输入的一个无法通过 parseFloat 或 parseInt ，您的当前代码最终会计算小计的 NaN 。

Arithmetic operations on numbers with the value NaN almost always result in NaN (NaN + 5 will result in NaN.) That means, if only one of the input cannot be parsed by parseFloat or parseInt, your current code would end up calculating NaN for the subtotal.

评论中已经提到过（主要是 Felix ），但我认为值得强调因为这些是重要的问题：

It's already been mentioned in the comments (by Felix mostly) but I think it's worth the emphasis as these are important concerns:

• 始终将 radix 参数传递给 parseInt function;


• 不要对变量名使用 class ：它是保留的（未使用，但保留）关键字;


• 不要指望小计 完全准确，除非你以美分计算。

• Always pass the radix argument to the parseInt function;
• Do not use class for variable names: It's a reserved (not used, but reserved) keyword;
• Don't expect the subtotal to be perfectly accurate unless you do your calculations in cents.

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