为什么在JavaScript中将value与undefined的比较返回false？

Question

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); 

Answer 1

This is how works in JavaScript.

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

So, while you compare it forces to check like:

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

And thus,

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