Javascript 为什么 console.log(typeof "Not a Number" - "Number") 打印 'NaN'？

Question

我知道并承认

console.log(typeof NaN) // 'number'

但是我需要帮助来理解

的逻辑

console.log(typeof "Not a Number" - "Number") // 'NaN'

看着这个

console.log("NaN is normal" - "normal" + "special") // NaNspecial

我看到 "NaN is normal" - "normal" 给出了 NaN（这是一个 number 类型），然后在串联之前将其转换为 string。

Answer 1

参见operator precedence。 typeof的优先级为17，减法的优先级为14。所以

console.log(typeof "Not a Number" - "Number") // 'NaN'

相当于：

console.log(typeof "Not a Number" - "Number") // original line
console.log((typeof "Not a Number") - "Number") // grouping; operator precedence
console.log(("string") - "Number")
console.log("string" - "Number")
// A string can't be meaningfully subtracted from another string, so the result is NaN
console.log(NaN)

类似地，-和+具有相同的优先级，并且从左到右工作，所以最终代码相当于：

console.log("NaN is normal" - "normal" + "special") // original line
console.log(("NaN is normal" - "normal") + "special")
console.log((NaN) + "special")
// NaN gets coerced to a string and concatenated:
console.log("NaNspecial")

Javascript 为什么 console.log(typeof "Not a Number" - "Number") 打印 'NaN'？

Javascript why console.log(typeof "Not a Number" - "Number") prints 'NaN'?

javascript

types

typeof

nan