Array.filter() with async arrow function

Question

I am trying to filter my array using Array.filter() function, however I came across this issue. I need to call some other function inside the filter function asynchronously. However the array is not changing its value based on the conditions that I define in the function.

const filterWithEmail = data.filter(async (doc) =>
{
   const findUser = await UserService.findUser(doc.id).catch(err => {});
   if (findUser)
   {
      const { email } = findUser;
      return regexFilter ? regexFilter.test(email.normalize("NFKC")) : false;
   }
});

This code doesn't affect the data array at all for some reason. Any suggestions what I'm doing wrong?

Thank you in advance.

.filter expects synchronous predicate. Since you provide an async function, it just takes the returned promise as a truthy value and assumes everything passes. — VLAZ
– VLAZ, Commented Nov 10, 2020 at 14:36

Quentin · Accepted Answer · 2020-11-10 14:54:16Z

filter expects the return value of the callback to be a boolean but async functions always return a promise.

You don't know if you want to return true or false in time to tell filter which it is.

What you possibly want to do is:

map the data from data to { keep: true, original_data: data } (using an async callback)
Pass the resulting array of promises to Promise.all
await the return value of Promise.all
filter that array with: .filter(data => data.keep)
Get the original objects back with .map(data => data.original_data)

Something along these lines (untested):

const filterWithEmail = (
  await Promise.all(
    data.map(async (data) => {
      const findUser = await UserService.findUser(doc.id).catch((err) => {});
      let keep = false;
      if (findUser && regexFilter)
        keep = regexFilter.test(email.normalize("NFKC"));
      return { data, keep };
    })
  )
)
  .filter((data) => data.keep)
  .map((data) => data.data);

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