How to return a boolean in a promise?

Question

Inside submitButton event, I would like to check if there are any users registered or not.

Suppose there are 5 users registered:

 getUsers = function () {
    return new Promise(function (resolve, reject) {
      resolve(5);
      reject(0);
    });
  }

Inside the checkregisteredUsers, I call getUsers and then I check if there are any users. If there are any users, then return true, otherwise false. I set state of UsersFound.

  checkRegisteredUsers = () => {
    return this.getUsers().then(result => {
      return this.setState({ UsersFound: result > 0 ? true : false };
    }).catch(reason => {
      console.log(reason);
    });
  }

Now, inside the function submitButton, I would like to check for true or false, if there were users registered.

submitButton = () => {
      this.checkRegisteredUsers().then(result => {
      this.setState({ UsersFound: result > 0 ? true : false });
    });

    if(!this.state.UsersFound){
    return; **//exit code here and the rest of the code below should not execute and show the dialog box**
    }
    // run some other code
  }

When I call submitButton, the state of UsersFound is not set yet. If I click a second time then it is set. How can I set it inside a promise? Or How can I check for true/false inside the submitButton using promises?

EDIT:

The question is: how can I return a boolean from a promise as I would like to check something like this:

submitButton = () => {
 if(this.checkRegisteredUsers()){
  return; //the rest of the code below should not execute
}
 //if check users is true then the code below should not execute
 // some other code
}

I don't want a promise returned, I want a boolean.

remove the `then` and `catch` callback from the `checkRegisteredUsers` method. I recommend using `asyn/await` statements to handle async functions as well. — Mahdi Tohidloo, May 09 '20 at 21:10
are you suggesting like this: `checkRegisteredUsers = () => { return this.getUsers(); }` I still can't set the state of userChecks. it still returns false the first time I click on the button — Burre Ifort, May 09 '20 at 21:24
Does this answer your question? [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Roamer-1888, May 10 '20 at 14:28

rfestag · Answer 1 · 2020-05-10T17:51:25.093

Long story short: You cannot make a function that returns a promise return a boolean. If you have some asynchronous action (like a fetch to get users from some service), that is inherently asynchronous. You cannot have such a function return a boolean, because by the time the function itself ends, you don't have the information to return true or false.

The closest you can get to that is using async/await. But async functions still only return Promise objects. You can make it look different using async/await, but it is just syntactic sugar. You are still effectively running all of the code after the await as though it were a function passed to then.

As an example, this:

//Some function that will resolve or reject.
const getUsers = () {
  return new Promise(function (resolve, reject) {
    resolve(5);
  });
}
const foundUsers = async () => {
  const users = await getUsers();
  return Boolean(users);
}
const LoggedIn = () => {
  const [usersFound, setUsersFound] = useState(false)
  const onSubmit = async () => {
    const found = await foundUsers();
    if (found) {
      //do stuff
    }
    setUsersFound(found)
  }
  return (
    <>
      <button onClick={onSubmit}>Submit</button>
      { usersFound ? <Dialog /> : <WhateverElse /> }
    </>
  )
}

Is the same as this:

const LoggedIn = () => {
  const [usersFound, setUsersFound] = useState(false)
  const onSubmit = () => {
    foundUsers().then((found) => {
      if (found) {
        //do stuff
      }
      setUsersFound(found)
    })
  }
  return (
    <>
      <button onClick={onSubmit}>Submit</button>
      { usersFound ? <Dialog /> : <WhateverElse /> }
    </>
  )
}

the intention is to not execute the rest of the code insde submit button if users were found. How can I do that? That is why I am trying to use the true false here. — Burre Ifort, May 10 '20 at 06:41
I replaced the answer to something I think better answers what you're asking "how can I return a boolean from a promise". Hopefully this makes more sense. — rfestag, May 10 '20 at 17:52

How to return a boolean in a promise?

1 Answers1