Expo FileSystem.writeAsStringAsync doesn’t act like a Promise Code Answer

I’m using the function Expo.writeAsStringAsync() (docs: https://docs.expo.io/versions/latest/sdk/filesystem/#filesystemwriteasstringasyncfileuri-contents-options). I noticed that it needs different times with different size files (as expected), but there is no way to know when it has finished, because it returns nothing. So, if I have to access the file just after writing it, I could find it empty, because maybe it’s still writing.

Is there any way to receive an answer when it has completed? like a normal promise-then-catch?

P.S.: I’ve tried to promisify it, but without success.

Answer

I can see that the API docs are misleading because they don’t specify the return type.

In fact that API call is defined as async function (see source-code):

export async function writeAsStringAsync(
  fileUri: string,
  contents: string,
  options: WritingOptions = {}
): Promise<void> {
  // ...
}

Every async function returns a Promise (you can see in TypeScript signature above it says Promise<void>).

This means you can use the returned Promise and await for it or use .then() to wait for the moment of filesystem call to complete.

await Expo.writeAsStringAsync(fileUri, contents, options);
// do something when write is finished 
// you can catch errors with try/catch clause

or

Expo.writeAsStringAsync(fileUri, contents, options).then(
  () => { /* do something when write is finished */ }
).catch(err => { /* handle errors */ }