js|isNaN() 前端|javascript

【js|isNaN()】返回值:如果给定值为 NaN则返回值为true；否则为false。
NaN值的产生:
当算术运算返回一个未定义的或无法表示的值时,NaN就产生了
NaN并不一定用于表示某些值超出表示范围的情况
将某些不能强制转换为数值的非数值转换为数值的时候,也会得到NaN
如果isNaN函数的参数不是Number类型,isNaN函数会首先尝试将这个参数转为数值,
然后才会对转换后的结果进行isNaN判断.
例子:

console.log(isNaN(NaN)); //true console.log(isNaN(undefined)); //true console.log(isNaN({})); //true console.log(isNaN(0/0)); //true console.log(isNaN('88,99')); //true console.log(isNaN()); //true console.log(isNaN('123abc')); //true console.log(isNaN('hello'))//true console.log(isNaN(new Date().toString())); //trueconsole.log(isNaN([])); //false console.log(isNaN(false)); //false console.log(isNaN(true)); //false console.log(isNaN(null)); //false console.log(isNaN('')); //false console.log(isNaN(' ')); //false console.log(isNaN('8')); //false console.log(isNaN('8.9')); //false

有用的特殊行为
有许多方式来看待isNaN()：如果isNaN(x)返回false，那么x在任何算数表达式中都不会使表达式等于NaN；如果返回true，x会使所有算数表达式返回NaN。这就意味着，在JavaScript中，isNaN(x)==true等价于x-0=NaN(在JavaScript中 x-0 == NaN 总是返回false，所以你不用去测试它)。实际上， isNaN(x), isNaN(x - 0),isNaN(Number(x)), Number.isNaN(x - 0),和Number.isNaN(Number(x)) 的返回值都是一样的并且在JavaScript中isNaN(x)是这些表达式中最短的表达。
举个例子，可以利用这个特殊行为来检测函数的参数是可运算的（可以像number一样进行加减乘除等运算）。如果不可运算，则可赋予这个参数一个默认的值或其他合适的内容。这样，就可以得到一个隐式转换参数值的函数，而这得益于Javascript的全功能性。
例子

function increment(x) { if (isNaN(x)) x = 0; return x + 1; }; // The same effect with Number.isNaN(): function increment(x) { if (Number.isNaN(Number(x))) x = 0; return x + 1; }; // In the following cases for the function's argument x, // isNaN(x) is always false, although x is indeed not a // number, but can be used as such in arithmetical // expressions increment(""); // 1: "" is converted to 0 increment(new String()); // 1: String object representing an empty string is converted to 0 increment([]); // 1: [] is converted to 0 increment(new Array()); // 1: Array object representing an empty array is converted to 0 increment("0"); // 1: "0" is converted to 0 increment("1"); // 2: "1" is converted to 1 increment("0.1"); // 1.1: "0.1" is converted to 0.1 increment("Infinity"); // Infinity: "Infinity" is converted to Infinity increment(null); // 1: null is converted to 0 increment(false); // 1: false is converted to 0 increment(true); // 2: true is converted to 1 increment(new Date()); // returns current date/time in milliseconds plus 1// In the following cases for the function's argument x, // isNaN(x) is always false and x is indeed a number increment(-1); // 0 increment(-0.1); // 0.9 increment(0); // 1 increment(1); // 2 increment(2); // 3 // ... and so on ... increment(Infinity); // Infinity// In the following cases for the function's argument x, // isNaN(x) is always true and x is really not a number, // thus the function replaces it by 0 and returns 1 increment(String); // 1 increment(Array); // 1 increment("blabla"); // 1 increment("-blabla"); // 1 increment(0/0); // 1 increment("0/0"); // 1 increment(Infinity/Infinity); // 1 increment(NaN); // 1 increment(undefined); // 1 increment(); // 1// isNaN(x) is always the same as isNaN(Number(x)), // but the presence of x is mandatory here! isNaN(x) == isNaN(Number(x))// true for every value of x, including x == undefined, // because isNaN(undefined) == true and Number(undefined) returns NaN, // but ... isNaN() == isNaN(Number())// false, because isNaN() == true and Number() == 0