Enhance existing functionalities in library with type augmentation strategy

I am interested in creating a library that includes a custom matcher function in TypeScript using Vitest. After following the documentation and adding a vitest.d.ts file with the necessary augmentations, everything appears to be working well. However, I have encountered an issue where other projects importing my library are not aware of the augmentation, resulting in a TypeScript error when trying to call the new matcher:

expect(data).toMatchTheTypicalExpectations()
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error TS2339: Property 'toMatchTheTypicalExpectations' does not exist on type 'Assertion<Data>'.

While individuals could manually add the augmentation themselves, it would be much more efficient to export the augmentation along with the library for better accessibility.

typescriptvitest

Answer №1

After experimenting with multiple approaches, I finally discovered a resolution to my issue.

To start, I included a file named vitest.ts (not d.ts!) in the source folder of my library, which contained the following augmentations:

export * from "vitest"

declare module 'vitest' {
  interface Assertion<T> {
    toMatchTheTypicalExpectations(): T
  }
  interface AsymmetricMatchersContaining {
    toMatchTheTypicalExpectations(): unknown
  }
}

Next, I inserted the following line into my main file named index.ts

import "./vitest.js"

When building the types for the library, the vitest.d.ts file is included along with its reference in index.d.ts, ensuring it is recognized by any user of the library.

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