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Conditional type does not narrow union type

See original GitHub issue

TypeScript Version: 3.2.2

Search Terms: conditional types, unions, narrowing


    interface A<T> {
        value: T;

    interface Specification {
        [key: string]: Array<any> | Specification;

    type Mapping<S extends Specification> = {
        [key in keyof S]: S[key] extends Array<infer T> ? A<T> : Mapping<S[key]>
        // Error                                                         ^^^^^^
        // Type 'S[key]' does not satisfy the constraint 'Specification'.
        //   Type 'Specification[key]' is not assignable to type 'Specification'.
        //     Type 'any[] | Specification' is not assignable to type 'Specification'.
        //       Type 'any[]' is not assignable to type 'Specification'.
        //         Index signature is missing in type 'any[]'.

Expected behavior: No error. “Leafs” of the Specification tree, which have type Array<T> (for some T) should be mapped to A<T>, while non-leaf properties should be recursively mapped.

Actual behavior: In the right-hand side of the conditional type, S[key] is not narrowed to Specification, even if the complete type of S[key] is Array<any> | Specification and the Array<any> case is catched in the left-hand side.

Playground Link: link

Related Issues: some similar issues related to conditional types, but I’m not sure whether this is a duplicate of any of them.

Issue Analytics

  • State:open
  • Created 5 years ago
  • Comments:9 (2 by maintainers)

github_iconTop GitHub Comments

jack-williamscommented, Jan 3, 2019

Conditional types do not produce substitution types for the false branch of the conditional (a.k.a do not narrow in the false branch).

There was an attempted fix here: #24821, however this was closed.

TotallyNotChasecommented, Jan 7, 2021

Alongside the narrowing mentioned here, Is it possible to add narrowing of union object types using in operator for conditional types?


type Foo = string | { type: string };

type Bar<T extends string> = T;

type Qux<T extends Foo> = {
    [K in keyof T]: T[K] extends string ? Bar<T[K]> : T[K] extends { type: string } ? Bar<T[K]['type']> : never;

It’d be nicer if this was possible instead-

type Qux<T extends Foo> = {
    [K in keyof T]: 'type' in T[K] ? Bar<T[K]['type']> : Bar<T[K]>;
Read more comments on GitHub >

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