Type the result of Promisse

Question

Is possible to avoid this cast:

(await getCryptocurrencyMap(1, 5)) as Cryptocurrency[]

And return the type on getCryptocurrencyMap?

Full code:

type Cryptocurrency = {
    id: number
    symbol: string
    name: string
}

const getCryptocurrencyMap = (start: number, limit: number) =>
  new Promise(async (resolve, reject) => {
    try {
      const response = await axios.get(`${BASE_URL}/v1/cryptocurrency/map`, {
        params: { start, limit },
        headers: {
          'X-CMC_PRO_API_KEY': COINMARKETCAP_APIKEY,
        },
      })

      resolve(response.data.data)
    } catch (error) {
      reject(error)
    }
  })

export const getCryptocurrencies = async () => {
  const arr = (await getCryptocurrencyMap(1, 5)) as Cryptocurrency[]

  return arr.map(({symbol, name}) => ({ [symbol]: name }))
}

Answer 1

Yes -- use generics:

new Promise<Cryptocurrency[]>((resolve, reject) => {
   // resolve can only be used with `Cryptocurrency[]` now
});

However, the new Promise wrapper seems redundant here. Just return the axios response, which also allows for generics:

const result = await axios.get<Cryptocurrency[]>(url, opts);
const currencies = result.currencies; // data is Cryptocurrency[] 

Note that this is pure Typescript runtime casting. It'll not guarantee you that the actually returned result of the API is of the specified type.