The "keyof typeof Module" function is not functioning properly with interfaces

I am trying to create a custom dynamic type that represents a union of interface symbols from a file called "MyInterfaces.ts"

export interface SomeInterfaceA {}
export interface SomeInterfaceB {}
...
export interface SomeInterfaceZ {}

The definition I am using is as follows:

import * as AllMyInterfaces from "./MyInterfaces"

type InterfaceNames = keyof typeof AllMyInterfaces
const test: InterfaceNames = "SomeInterfaceA"
//^^^^^^^^^ TS2322: Type 'string' is not assignable to type 'never'.

However, I am getting an error from the compiler stating: Type 'string' is not assignable to type 'never'

It seems like the result of "typeof AllMyInterfaces" does not contain any interface symbol. How can I properly define my Union type in order to resolve this issue? Thank you for your help.

Answer №1

When exporting types (like interfaces), they do not actually contribute anything to the exported module. As a result, the keyof type operator disregards "type fields" for star imports.

This concept becomes clearer when illustrated with an example module:

export const ABC = 123;
export type Def = number;

If we were to use the following code, it would yield this output:

import * as MyModule from './module';
Object.keys(MyModule); // ['ABC']

Therefore, it is logical that the keyof operator does not consider the types.

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