Javascript 返回 NaN
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Javascript returning NaN
提问by csm232s
I'm attempting to display a subtotal each time a customer enters a quantity. However, when I loop through my inputs, I get a NaNas the total. I believe it may be the way I'm declaring the subtotalvariable in my script, but I haven't come across this before:
每次客户输入数量时，我都试图显示小计。但是，当我遍历我的输入时，我得到 aNaN作为总数。我相信这可能是我subtotal在脚本中声明变量的方式，但我以前从未遇到过：
$('.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);
});
回答by Bryan Menard
parseFloatand parseIntcan return NaNif the first character of the string cannot be converted to a number.
parseFloat如果字符串的第一个字符无法转换为数字，则parseInt可以返回NaN。
So, I would safeguard against it like this (NaNis a falsyvalue):
所以，我会像这样防止它（NaN是一个虚假的价值）：
price = parseFloat(price) || 0;
qty = parseInt(qty, 10) || 0;
Arithmetic operations on numbers with the value NaNalmost always result in NaN(NaN + 5will result in NaN.) That means, if only oneof the input cannot be parsed by parseFloator parseInt, your current code would end up calculating NaNfor the subtotal.
对具有该值的数字进行算术运算NaN几乎总是导致NaN(NaN + 5将导致NaN。) 这意味着，如果只有一个输入不能被parseFloator解析parseInt，您当前的代码最终将计算NaN小计。
It's already been mentioned in the comments (by Felixmostly) but I think it's worth the emphasis as these are important concerns:
评论中已经提到了（主要是Felix），但我认为值得强调，因为这些是重要的问题：
- Always pass the
radixargument to theparseIntfunction; - Do not use
classfor variable names: It's a reserved (not used, but reserved) keyword; - Don't expect the subtotal to be perfectlyaccurateunless you do your calculations in cents.
- 始终将
radix参数传递给parseInt函数； - 不要
class用于变量名：它是一个保留（未使用，但保留）关键字； - 不要期望小计完全准确，除非您以美分进行计算。
