jQuery：parseInt（）和Number在它应该是数字时返回NaN

Question

HTML

double&

JS

<div class="slider-container slide-0" num-slides="5" rel="0"></div>

目前：

当我在Google Chrome 64.0中运行该程序时，parseInt和Number不会返回数值。但是，当我在dev工具控制台中输入代码时：

var maxNum = parseInt($('.slide-0').attr("num-slides"));
// maxNum = NaN

var maxNum = parseInt($('.slide-0').attr("num-slides").value);
// "Uncaught TypeError: Cannot read property 'value' of undefined"

var maxNum = parseInt($('.slide-0').attr("num-slides").val());
// Uncaught TypeError: Cannot read property 'val' of undefined

var maxNum = Number($('.slide-0').attr("num-slides"));
// maxNum = NaN

为什么会出现这种情况？