Why is TypeScript throwing the `void is not assignable` error?

Question

I'm new to TS so the question might be trivial to some of You, however I'm kind of missing something,

I'm trying to create a function that will return an array containing the users geolocation data like lingitude, latitude. I want this to be correctly covered by the TS type checking magic.

export const getUsersGeolocationData = (): [number,number] => {
    const navigatorResult: [number, number] = navigator.geolocation.getCurrentPosition(position =>{
        return [position.coords.latitude, position.coords.longitude];
    })
    return navigatorResult
}

console.log(getUsersGeolocationData())

My understanding: I'm letting TS know that I expect the [number, number] array as the result of getUsersGeolocationData() but I keep receiving the Type 'void' is not assignable to type '[number, number]'

Can somebody help me with understanding and fixing the issue ? :) Best regards !

A function's "signature" is the types on its parameters and return type. The return type is the important one for your error: getCurrentPosition doesn't return anything. — Nicholas Tower
– Nicholas Tower, Commented Dec 1, 2020 at 21:56
Oh, it gives undefined / void . So how Can i get it working ? — AdamKniec
– AdamKniec, Commented Dec 1, 2020 at 21:59

Pytth · Accepted Answer · 2020-12-01 22:04:37Z

1

If this is the API I am thinking it is, it looks like getCurrentPosition accepts a callback as the first argument that doesn't returnanything.

If you want this to work, perhaps you should consider turning it into a promise and consuming it that way.

export const getUsersGeolocationData = (): Promise<[number,number]> => {
    return new Promise((resolve, reject) => {
        navigator.geolocation.getCurrentPosition(position =>{
            resolve([position.coords.latitude, position.coords.longitude])
        })
    })
}

getUsersGeolocationData().then(res => console.log(res))

edited Dec 1, 2020 at 22:04

answered Dec 1, 2020 at 21:57

Pytth

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1 Comment

AdamKniec Over a year ago

So it's the matter of the async nature of the getUsersGeolocationData that i couldn't get it working and it had nothing to do with TS .... Thank You so much for the explanation :)

Anatoly · Accepted Answer · 2020-12-01 22:04:55Z

1

You should turn a call of this function into async function in order to get a result:

export const getUsersGeolocationData = (): Promise<[number,number]> => {
    return new Promise(resolve, reject) => {
      navigator.geolocation.getCurrentPosition(position =>{
        resolve([position.coords.latitude, position.coords.longitude]);
      })
    });
}

...
getUsersGeolocationData().then(coords => console.log(coords))

answered Dec 1, 2020 at 22:04

Anatoly

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Comments

Tomasz Gawel · Accepted Answer · 2020-12-01 22:23:49Z

1

const getCurrentPosition = navigator.geolocation.getCurrentPosition.bind(navigator.geolocation);
const getCurrentPositionAsync = () => new Promise(getCurrentPosition);
export const getUsersGeolocationData = async () => {
    const position = await getCurrentPositionAsync();
    return [position.coords.latitude, position.coords.longitude] as const;
}

// then you call it:
getUsersGeolocationData().then(result => console.log(result));

// or from inside async function:
async function main() {
    console.log(await getUsersGeolocationData());
}

answered Dec 1, 2020 at 22:23

Tomasz Gawel

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