Javascript逻辑空赋值等价

Question

我正在访问 MDN article for Logical Nullish Assignment . MDN 提供的等效版本是 x ?? (x = y)这还不够清楚，需要深入挖掘。
这段代码是:

let x = null;

x ??= 12;

相当于:

let x = null;

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

Answer 1

是的，它只分配 x是无效的。

x ?? (x = y)

或者

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

是 不是 相当于

x = x ?? y

存在差异，您可以使用 const 进行验证。而不是 let .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)
}