What is the process of defining an object enum in a declarations document?

Here is a sample javascript file named test.js:

const someType = {
  val1: "myvalue",
  val2: "myothervalue"
};

function sampleFunction(param) {
  return 1;
}

function sampleFunction2(param) {
  return 2;
}

export {someType, sampleFunction, sampleFunction2};

Alongside it, we have a definitions file called test.d.ts:

declare module "test" {
  // defining an enum-like object within the module
  export type someType = {
    val1: 'myvalue',
    val2: 'myothervalue',
  }

  export function sampleFunction(param1: someType): number;
  export function sampleFUnction2(param1: someType): number;
}

The question arises on how to correctly define the object enum in the definitions file.

import sampleFunction, someType from 'test';

console.log(sampleFunction(someType.val1)); /// encountered issue here with someType 

The above code snippet faces issues as someType is not recognized as a valid value. Using { someType } while importing results in an error as it's a type being treated as a value.

Check out this link for more details.

Answer №1

Expressions in types are not allowed. To address this, it is recommended to declare a constant with the desired type as constants will be declared and present at runtime:

declare module "test" {
  export const someType: {
    val1: 'myvalue',
    val2: 'myothervalue',
  }
  type someTypeValue = typeof someType[keyof typeof someType];
  export function sampleFunction(param1: someTypeValue): number;
  export function sampleFUnction2(param1: someTypeValue): number;
}
// usage.ts
import { sampleFunction, someType} from 'test';

console.log(sampleFunction(someType.val1));
console.log(sampleFunction('myvalue')); // will also work

In the above example, the constant is exposed directly allowing any string value that is a member of sampleType to be passed in.

An alternate approach would involve using an enum instead, which conceals the string values within the enum:

declare module "test" {
  export enum someType {
    val1 = "myvalue",
    val2 = 'myothervalue'
  }
  export function sampleFunction(param1: someType): number;
  export function sampleFUnction2(param1: someType): number;
}
// usage.ts
import { sampleFunction, someType} from 'test';

console.log(sampleFunction(someType.val1));
console.log(sampleFunction('myvalue')); // error

Either version can be suitable depending on the specific requirements you have.

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