Typescript - whichever type of those imported from a module

Question

I have something like:

import * as Types from '../schema/types';

and I wanna do something like:

let a: Types;

indicating that a has to be one of the multiple types exported from types.ts . How can I do that? (Yes it's a graphql server)

Answer 1

Looks like you are intention is to use union types . For typescript union types you can refer below sample example -

Scenario 1: Construct individual types in other module and use union during the usage:

types.ts

type a = {
    name: string;
}

type b ={
    name: number;
}

export type {a, b}

You can use it like:

import {a,b} from './types';

let a: a|b;

Or

import * as types from './types';

let a: types.a|types.b;

Scenario 2: Construct union types in other module and use in calling side:

types.ts

type a = {
    name: string;
}

type b ={
    id: string;
}

export type types = a|b

Use it

import {types}  from './types';

let a: types;

For using the union types in graphql use the example mentioned here -

Update: After seeing your comments regarding getting all the types the answer is NO . The interfaces and types fall under the type space which dont even exist in the JS world and they are just compiler hints for objects to have specific structure. In other words you even cannot do the Object.Keys/ Object.entries to get all the types from the module and assign them like you do in classes. These dont even exist in the run time.

Your best bet would be to extract them from the module and convert to union types or construct the union type in the types.ts file and use them in the main file.

Answer 2

I can't think of a way to do it automatically, but what you need is a union type .

Assuming you have the following types declared in the ../schema/types :

interface Foo {
 ...
}

interface Bar {
 ...
}

imported with import * as Types from '../schema/types';

You can create a union type like:

type GqType = Types.Bar | Types.Foo;

Then you can create a like:

let a: GqType;