打字稿:`| 的含义[T]`-用于数组中元素成员资格的编译时检查的约束 [英] TypeScript: Meaning of `| [T]`-constraint for compile-time checks of element membership in arrays

问题描述

我想编写一个函数 f，它接受 A 类型的单个参数，其中 A 必须是一个包含元素的数组某种类型的T.为简单起见，假设 T = 'ok'(字符串 'ok' 的单一类型).

I want to write a function f that accepts a single argument of type A, where A must be an array that contains an element of a certain type T. For simplicity, assume that T = 'ok' (singleton type of the string 'ok').

使用来自这个答案的想法，我得到了以下初步解决方案:

Using the idea from this answer, I obtained the following preliminary solution:

function f<
  E,
  A extends (ReadonlyArray<E> | [E]) & {[K in keyof A]: {[T in K]: 'ok'}}[number]
>(a: A) {}

正如上面引用的答案，它确实有效.

As in the answer quoted above, it indeed works.

但我无法理解|[E]-part，所以我决定检查是否可以删除它，或者用另一种类型替换它:

But I could not make any sense of the | [E]-part, so I decided to check whether I can remove it, or replace it by yet another type:

function f<
  E, 
  X, 
  A extends (ReadonlyArray<E> | [X]) & {[K in keyof A]: {[T in K]: 'ok'}}[number]
>(a: A) {}

这也有效，即它可以区分带有或不带有元素 'ok' 的数组:

This also works, i.e. it can differentiate between arrays with or without element 'ok':

f(['a', 'b', 'ok', 'z']) // compiles, good
f(['a', 'b', 'z'])       // does not compile, good

我的问题是，如果我删除 [X] 部分，我无法理解为什么它不起作用.它似乎与代码中发生的任何事情完全无关:

My problem is that I can't understand why it doesn't work if I remove the [X] part. It seems completely unrelated to anything what's going on in the code:

• [X] 的元数与元组的元数不匹配
• 类型 X 实际上并没有绑定到任何东西，它没有包含 E 或 A 或 string<的边界/code> 或 'ok' 或其他任何东西.

• Arity of [X] does not match the arity of the tuples
• The type X isn't actually tied to anything, it has no bounds containing either E or A or string or 'ok' or anything else.

什么是<代码>|[X] 到底在做什么?

What is the | [X] doing, exactly?

推荐答案

举个简单的例子:

function foo<T extends number[]>(t: T): T { return t }
foo([1, 2, 3]) // number[]

将推断出一个数组返回类型.

will have an array return type inferred.

function foo2<T extends number[] | [number]>(t: T): T { return t }
foo2([1, 2, 3]) // [number, number, number]

<代码>|[编号] 强制 TypeScript 进行推断一个 tuple 而不是数组，不使用 as const (隐藏的编译器规则，如果你愿意的话).

| [number] forces TypeScript to infer a tuple instead of an array, without using as const (a hidden compiler rule, if you will).

您甚至可以通过为扩展基本类型的数组项添加类型参数 R 来保持数字文字的范围:

You can even keep the number literals narrow by adding a type parameter R for the array items extending a primitive type:

function foo3<R extends number, T extends R[] | [R]>(t: T): T { return t }
foo3([1, 2, 3]) // [1, 2, 3]

游乐场

这篇关于打字稿:`| 的含义[T]`-用于数组中元素成员资格的编译时检查的约束的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！