为什么 TypeScript 递归类型别名失败 generics

Question

I've got two type definitions describing almost the same thing:

 // compilation fails with `Type alias 'StringOrRecordOfStrings' circularly references itself.ts(2456)`
type StringOrRecordOfStrings = string | string[] | Record<string, StringOrRecordOfStrings>;

 // compiles without problems
type StringOrRecordOfStrings = string | string[] | { [property: string]: StringOrRecordOfStrings };

Is anyone able to explain why the first type definition doesn't compile?

• The recursive type aliases have been introduced in version 3.7 https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#more-recursive-type-aliases
• This SO answer explains how to use recursive type aliases Recursive Types in TypeScript
• for brevity I omitted the remaining key types from record, but it works even when we set the [property: string | number | symbol] [property: string | number | symbol] [property: string | number | symbol] .

Answer 1

The reason why Record<string, StringOrRecordOfStrings> is not allowed is that Record is a generic type , rather than a class or an interface.

There isn't a lot of clear documentation on this, but recursive type references for properties in objects, index signatures, and mapped types have been around for quite a while. The following works as early as TypeScript 3.3:

type Recursive = {p: Recursive} | {[p: string]: Recursive} | {[Key in 'a']: Recursive}

TypeScript 3.3 playground

This is why your second example type (with an index signature) checks.

TypeScript 3.7 extended the support for recursive references, as explained in this PR :

• Instantiations of generic class and interface types (for example Array<Foo> ).
• Array types (for example Foo[] ).
• Tuple types (for example [string, Foo?] ).

So now, also these three examples are valid:

type RecursiveCI = Promise<RecursiveCI>
type RecursiveT = [number, boolean, RecursiveT]
type RecursiveA = RecursiveA[]

I assume the sample is just test code, but you can get it to type check by using a helper interface like this:

type StringOrRecordOfStrings = string | string[] | Record<string, RecordInterface>

interface RecordInterface extends Record<string, StringOrRecordOfStrings> {}

TypeScript playground