为什么在 JavaScript 中将值与 undefined 进行比较会返回 false? [英] Why does comparing value with undefined returns false in JavaScript?

问题描述

我是 Javascript 的新手，我注意到当一个变量是 undefined 时，比较一个数字会返回 false，如下所示.为什么将 undefined 与数字进行比较会返回 false?

I am new to Javascript and I noticed when a variable is undefined, comparing a number returns false as below. Why does comparing undefined with numbers return false?

var a = undefined;
console.log(a < 10);
console.log(10 < a);
console.log(a == 10);

推荐答案

这就是 JavaScript 中的工作方式.

This is how works in JavaScript.

Number(undefined) // NaN
NaN == NaN // false
NaN < 0 // false
NaN > 0 // false

所以，当你比较它的力量时，检查如下:

So, while you compare it forces to check like:

Number(undefined) < 10
// undefined is coerced to check with number

因此，

undefined == 10 // false
undefined > 10 // false
undefined < 10 // false

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