Parzh from Ukraine's questions

有时为了简化具有复杂输入的函数签名，我们会定义一个“已知”复杂输入的集合，每个输入都与一个令牌相关联，并允许用户提供令牌。

我的函数适用于元组（这里简化为 3 元素元组），最初它没有任何编译器错误：

const TUPLE_LENGTH = 3

type Tuple = readonly [unknown, unknown, unknown]

function fn(tuple: Tuple): void {
    if (tuple.length !== TUPLE_LENGTH) {
        throw new Error(`Expected ${TUPLE_LENGTH} items, instead got ${tuple.length}`)
    }
}

尝试一下。

…但是在添加“已知”输入后，开始出现编译器错误：

const TUPLE_LENGTH = 3

type Tuple = readonly [unknown, unknown, unknown]

const knownTuples = {
    foo: [1, 2, 3],
    bar: ['a', 'b', 'c'],
    baz: [true, false, null],
} satisfies Readonly<Record<string, Tuple>>

type TupleName = keyof typeof knownTuples

function fn(tuple: Tuple | TupleName): void {
    if (typeof tuple === 'string') {
        tuple = knownTuples[tuple]
    }

    if (tuple.length !== TUPLE_LENGTH) {
        throw new Error(`Expected ${TUPLE_LENGTH} items, instead got ${tuple.length}`)
        //                                                                   ^^^^^^
        // Error: Property 'length' does not exist on type 'never'
    }
}

尝试一下。

由于某种原因，if (tuple.length !== TUPLE_LENGTH)在第一种情况下，它是 ，tuple: Tuple但在第二种情况下，它缩小到tuple: never。这是为什么？

我有以下逻辑：

interface ResultBase {
    readonly success: boolean
}

type Result<Payload = undefined> = ResultBase & (Payload extends undefined ? {
    readonly payload?: unknown
} : {
    readonly payload: Payload
})

function getResult<Payload = undefined>(payload: Payload): Result<Payload> {
    return {
//  ^^^^^^
        success: true,
        payload,
    }
}

尝试一下。

标记的return语句带有下划线并带有编译器错误：

Type '{ success: true; payload: Payload; }' is not assignable to type 'Result<Payload>'.
  Type '{ success: true; payload: Payload; }' is not assignable to type 'Payload extends undefined ? { readonly payload?: unknown; } : { readonly payload: Payload; }'.

我想知道，为什么错误消息中的条件类型不包含该success属性？不应该包含在吗ResultBase & …？