Unexpected type assertion result

TypeScript Version: 3.6.0-dev.20190801

Search Terms:

type assertion absolute import
type assertion import
type cast absolute import
type cast import
type assertion pick

Code

Repo: https://github.com/klemenoslaj/typescript-type-assertion

`generated/types.d.ts`

export interface TypeD {
  da: string;
  db: string;
  dc: string;
}

export interface Test { 
  a: string;
  b: string;
  c: string;
  d?: { __private: 'd' } & TypeD;
}

`src/index.model.ts`

import { Test } from 'generated/types';

type DType = NonNullable<Test['d']>;
export type Model = Pick<DType, keyof Exclude<DType, '__private'>>;

`src/index.ts`

import { Model } from './index.model';

export class A {
    value = <Model>{};
}

`dist/index.model.d.ts`

import { Test } from 'generated/types';
declare type DType = NonNullable<Test['d']>;
export declare type Model = Pick<DType, keyof Exclude<DType, '__private'>>;
export {};

`dist/index.d.ts`

export declare class A {
    value: Pick<{
        __private: "d";
    } & import("../generated/types").TypeD, "__private" | "da" | "db" | "dc">;
}

Expected behavior:

I would expect that dist/index.d.ts type definitions would result in:

import { Model } from './index.model';
export declare class A {
    value: Model;
}

In fact, this is the result of type definitions, if I change value = <Model>{}; to value: Model = <Model>{};.

Why doesn't typescript use Model directly when type asserting?

Actual behavior:

Results in:

export declare class A {
    value: Pick<{
        __private: "d";
    } & import("../generated/types").TypeD, "__private" | "da" | "db" | "dc">;
}

Playground Link:
I couldn't figure out how to compile declarations in playground, sorry :(

As an alternative please clone the following repo: https://github.com/klemenoslaj/typescript-type-assertion

Explanation:
This is quite a significant problem in monorepo environment.
What happens is that projects are compiled in the dist/ folder and they contain relative path like above (import("../generated/types")), which points from the src/ and not from dist/, effectively producing an error when projects start using one another.

Why doesn't typescript:

use the original path from the import (generated/types)?
use the type from assertion directly (Model)?

A generally feature of type aliases is that they can be expanded transparently by the compiler, so probably when you use the assertion the compiler does not think it needs to keep the name around.

Using an interface will probably get you the effect you desire:

export interface Model extends  Pick<DType, keyof Exclude<DType, '__private'>> {}

A generally feature of type aliases is that they can be expanded transparently by the compiler, so probably when you use the assertion the compiler does not think it needs to keep the name around.

Using an interface will probably get you the effect you desire:
export interface Model extends  Pick<DType, keyof Exclude<DType, '__private'>> {}

Yes, that works 👍

However, we cannot force people to use interfaces, therefore issue might still pop out causing frustrations, since it is not really straight forward problem.

Is there any other way?

EDIT

Furthermore, if types are generated by 3rd party, we are even out of control, and using the type directly from generated/types will result in the same problem.

I have update the repository accordingly:

export class A {
    value1 = <Model>{};
    value2 = <Test['e']>{};
}

Results in

export declare class A {
    value1: Pick<{
        __private: "d";
    } & import("../generated/types").TypeD, "__private" | "da" | "db" | "dc">;
    value2: {
        __private: "e";
    } & import("../generated/types").TypeD;
}

Navigation Menu

Repository navigation

Unexpected type assertion result #32653

`generated/types.d.ts`

`src/index.model.ts`

`src/index.ts`

`dist/index.model.d.ts`

`dist/index.d.ts`

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Unexpected type assertion result #32653

Description

generated/types.d.ts

src/index.model.ts

src/index.ts

dist/index.model.d.ts

dist/index.d.ts

Activity

dragomirtitian commented on Aug 1, 2019

klemenoslaj commented on Aug 1, 2019

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

`generated/types.d.ts`

`src/index.model.ts`

`src/index.ts`

`dist/index.model.d.ts`

`dist/index.d.ts`