What's isNaN

使用isNaN函数来判断一个变量是不是数字形式，通过下面代码可能会有更明确的认识：

function checkit(x) {
    let v = x;
    if (isNaN(x)) {
        v = 'Not a Number!';
    }else{
        v = Number(x);
    }
    console.log(x,'is',v);
 }

    checkit('123');
    checkit('-123');
    checkit('3.14');
    checkit('0.0314E+2');
    checkit(0xF);
    checkit('123F');
    checkit('abc');
    checkit(null);
    checkit(false);
    checkit(true);
    checkit(undefined);

输入结果：

123 is 123
-123 is -123
3.14 is 3.14
0.0314E+2 is 3.14
15 'is' 15
123F is Not a Number!
abc is Not a Number!
null 'is' 0
false 'is' 0
true 'is' 1
undefined 'is' 'Not a Number!'

需要注意的是：

123F is Not a Number!
null 'is' 0
false 'is' 0
true 'is' 1