为什么'let'声明符号将转换为 TypeScript 中的字符串索引签名？

Question

当使用let s = Symbol()将符号声明为对象的键时，如：

let a = { [s]:1 }

a 的类型会自动变为{[x:string]:number}，而尝试像a[s] 这样的索引时会出错：

类型 'symbol' 不能用作索引类型 (2538)

当使用const s = Symbol() 声明时，就像{[s]:number} 一样工作。

Answer 1

这是一个非常古老的问题。请参阅https://github.com/microsoft/TypeScript/issues/1863 和https://github.com/Microsoft/TypeScript/issues/24587。

后者的一个奇怪的评论是

we forbid symbols as indexers because we have no symbol index signatures (this was a hole we patched in 2.9 - previously it would assume the string indexer type, which was very incorrect), however unique symbols - a type associated with exactly one symbol are fine, since they refer to exactly one property.

在我看来你发现了一个他们在 2.9 中没有修复或从那时起恢复的案例......