Flow - Cannot call `data.map` because property `map` is missing in `FetchData` [1]

Question

Hi folks I'm trying to use flow with a custom useFetch hook. Here is the code:

import React, { useState, useEffect } from 'react'

function useFetch<FetchData>(
  promiseFn: () => Promise<FetchData>,
  args?: any[]
) {
  const [loading, setLoading] = useState(false)
  const [data, setData] = useState<?FetchData>()
  const [error, setError] = useState<?Error>()

  useEffect(() => {
    setLoading(true)

    promiseFn()
      .then(setData)
      .catch(setError)
      .finally(() => setLoading(false))
  })

  return {
    loading,
    data,
    error
  }
}
  
function fetchKeys() {
   return Promise.resolve([{ name: 'myKey' }])
}  
  
type KeysData = {
  name: string
}[]  
  
function MyKeysPage() {
	const { data, loading, error } = useFetch<KeysData>(fetchKeys)
    
    if(!data) return 'Loading...'

    return data.map(key => (<div>{key.name}</div>))
}

But it is throwing an error:

Cannot call data.map because property map is missing in FetchData [1].

You can take a better look here:

https://flow.org/try/#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

So I don't know how to fix that. Thanks for your help.

Radosław Miernik Radosław Miernik · Accepted Answer · 2019-05-16T08:23:12

It looks like Flow is not inferring the return type correctly. Explicitly typing useFetch helps:

function useFetch<FetchData>(
  promiseFn: () => Promise<FetchData>,
  args?: any[]
): {data: ?FetchData, loading: bool, error: ?Error} {
  // ...
}

Playground link.

Flow - Cannot call `data.map` because property `map` is missing in `FetchData` [1]

1 Answers