打字稿:从数组类型中检索元素类型信息 [英] Typescript: Retrieve element type information from array type

问题描述

假设我有一些数组类型 T[]，是否可以在另一个别名/接口中提取类型 T?例如，我的(假)理想代码如下:

Say I have some array type T[], is it possible to extract the type T within another alias / interface? For example my (fake) ideal code would be as follows:

// for illustration only...

type ArrayElement<T[]> = T;

// then, ArrayElement<string[]> === string

如果否，是否存在不允许使用此类运算符的一般类型理论原因?如果没有，我可能会建议添加它.

If no, are there general type theory reasons for not allowing such an operator? If no again, I might suggest it be added.

谢谢！

推荐答案

更新:基于@jerico 在下面的回答

以下类型别名将返回数组或元组中元素的类型:

The following type alias will return the type of the elements in an array or tuple:

type ArrayElement<ArrayType extends readonly unknown[]> = 
  ArrayType extends readonly (infer ElementType)[] ? ElementType : never;

所以这些例子会起作用:

So these examples would work:

type A = ArrayElement<string[]>; // string
type B = ArrayElement<readonly string[]>; // string
type C = ArrayElement<[string, number]>; // string | number
type D = ArrayElement<["foo", "bar"]>; // "foo" | "bar"
type E = ArrayElement<(P | (Q | R))[]>; // P | Q | R

type Error1 = ArrayElement<{ name: string }>; 
//                         ^^^^^^^^^^^^^^^^
// Error: Type '{ name: string; }' does not satisfy the constraint 'readonly unknown[]'.

说明

类型保护(尖括号中的位)ArrayType extends readonly unknown[] 表示我们期望类型参数 ArrayType 至少是一个只读数组(它还接受一个可变数组)以便我们可以查看它的元素类型.

The type guard (the bit in the angle brackets) ArrayType extends readonly unknown[] says that we expect the type parameter ArrayType to be at least a readonly array (it also accepts a mutable array) so that we can look at its element type.

这可以防止传入非数组值，如最后一个示例中所示，它可以防止 ArrayElement 永远返回 never.

This prevents passing in a non-array value, as in the final example, which prevents ArrayElement ever returning never.

注意 readonly unknown[] 是 TypeScript 3.4 添加的语法；对于早期版本，使用 ReadonlyArray.

Note that readonly unknown[] is syntax added in TypeScript 3.4; for earlier versions use ReadonlyArray<unknown>.

在右侧，条件表达式要求编译器在模式 readonly ElementType[] 中填写 ElementType 的值并返回 ElementType 如果可以，或者 never 如果不能.

On the right-hand side, the conditional expression asks the compiler to fill in the value of ElementType in the pattern readonly ElementType[] and return ElementType if it can, or never if it can't.

由于开头的类型保护意味着我们只会传递一个与此模式匹配的值，因此它保证始终匹配并且永远不会返回never.

Since the type guard at the beginning means we will only ever be passed a value which matches this pattern, it's guaranteed always to match and never to return never.

上一个回答

type ArrayElement<ArrayType extends readonly unknown[]> = ArrayType[number];

这篇关于打字稿:从数组类型中检索元素类型信息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！