Adding a method to an object with TypeScript: A step-by-step guide

In my current scenario, I am faced with a challenge where I need to test a function with a specific use of this. However, typescript poses constraints by either disallowing me from adding the method to the object, or if I define it as any, then my interface remains unused.

For example, consider the following code snippets:

import getMostPeersTorrentsFromDateRange from './index'
import Db from '../Db'

test('getMostPeersTorrentsFromDateRange', async () => {
  const db = Db() as any
  db.getMostPeersTorrentsFromDateRange = getMostPeersTorrentsFromDateRange
  const torrents = await db.getMostPeersTorrentsFromDateRange({startDate: '2019-03-03', endDate: '2019-03-08', limit: 100})
})

and in another file:

import {Torrent} from '../interfaces'

interface GetMostPeersTorrentsFromDateRange {
  (GetMostPeersTorrentsFromDateRangeSettings: {
    startDate: string,
    endDate: string,
    limit: number
  }): Promise<Torrent[]>
}

const getMostPeersTorrentsFromDateRange: GetMostPeersTorrentsFromDateRange = async function ({startDate, endDate, limit}) {
  return []
}

export default getMostPeersTorrentsFromDateRange

I am seeking a solution that minimizes code repetition. Is there a way to achieve this without defining

getMostPeersTorrentsFromDateRange
twice and without importing/exporting it again?

Answer №1

It seems like the error is originating from this particular line of code

db.getMostPeersTorrentsFromDateRange = getMostPeersTorrentsFromDateRange

This issue and its workaround are discussed here. In essence, TypeScript recognizes that db does not have the property

getMostPeersTorrentsFromDateRange
and therefore displays an error.

Instead of adding the property

getMostPeersTorrentsFromDateRange
, I recommend utilizing the call method of all JavaScript functions. This method allows you to specify any desired this context and maintains type safety.

getMostPeersTorrentsFromDateRange.call(db, {startDate: '1', endDate: '2', limit: 3})

Check out this playground example

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