Javascript逻辑null赋值等价

Question

我正在访问MDN关于逻辑零赋值的文章。MDN提供的等价版本是x ?? (x = y)，这还不够清楚，需要进一步挖掘。

这个代码是吗

let x = null;
x ??= 12;

相当于：

let x = null;
if (x === null || x === undefined) {
x = 12;
}

Answer 1

是的，它只在x为null时赋值。

x ?? (x = y)

或

if (x === null || x === undefined) {
x = 12;
}

它NOT相当于

x = x ?? y

存在差异，您可以使用const而不是let来验证这一点。这样，重新分配将导致错误。这也适用于其他逻辑赋值，如AND(&&=(和OR(||=(赋值运算符。

// no const reassignment error here  
// because a || (a = 'updated') doesn't evaluate the second expression
// if it was a = a || 'updated', it would throw an error
const a = 'initial'
a ||= 'updated' 
console.log(a)
const b = null
b &&= 'updated'
console.log(b)
const c = 'initial'
c ??= 'updated' // c ?? (c 
console.log(c)
// The below 3 examples will evaluate the second expression 
// and const reassignment error is thrown
try {
const d = null
d ||= 'updated'
} catch (e) {
console.log("|| Error: " + e.message)
}
try {
const e = 'initial'
e &&= 'updated'
} catch (e) {
console.log("&& Error: " + e.message)
}
try {
const f = null
f ??= 'updated'
} catch (e) {
console.log("?? Error: " + e.message)
}

Answer 2

逻辑零赋值(x？？=y(运算符仅在x为零(null或未定义(时赋值