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'

但是我需要帮助理解

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 NaN (这是一个 number 类型)，然后将其转换为 string在串联之前。

Answer 1

见 operator precedence . typeof优先级为 17，减法优先级为 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)

同样， -和 +具有相同的优先级，并且从左到右工作，因此最终代码等效于:

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")