Question 174086 in Javascript Question posted in
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Javascript – Execute a function only when promise is rejected

I am trying to create a createUser function that should only create a user if no user is found with the chosen username (that is, the user does not exist). Also this function should return a promise that i would later handle.

This function findbyUsername is supposed to return a user with the username ONLY if the user exists.
I am banking on the case where the user does NOT exist because ONLY then can a NEW user with that username be created. (you cannot have two users with same username)

const findByUsername = (name) => {
  return new Promise((res, rej) => {
    pool.query(
      "SELECT * FROM users WHERE username = $1",
      [name],
      (error, results) => {
        if (error) {
          rej("no user found");
        }
        console.log(results.rows);
        res(results.rows[0]);

        //return results.rows[0];
      }
    );
  });
};

The function above resolves if the username exists and rejects if not. NOW HERES THE THING….. I want to chain this function to another function that will create a user ONLY when findByUsername is rejected. Pls how do i get my createUser function to run only when findbyUsername is rejected. Note that createUser will also return a new promise that will be resolved or rejected and so need to be handled

Tags:

2

Answers


  1. 0

    You can use .then and .catch chaining to only execute the function.

    findByUsername("")
.then(() => doSomethingElse())
.catch(() => createUser())

    However I would recommend to do it like this using a guard state.

    
const user = await findByUsername("").catch(() => undefined); // return user or undefined if err

if (user === undefined){
 return createUser();
}

return doSomethingElse();
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  2. 0

    The same way as you would handle any rejection – with a catch method if chaining promises or a try...catch if you’re working with async/await.

    For the purpose of demonstration my equivalent of your method just checks for my username and resolves with an id of 1 if that’s the input.

    const findByUsername = (name) => {
  return new Promise((res, rej) => {
    if(name === "jamiec"){
      res(1)
    }
    else{
       rej("No user found");
    }
  });
};

// async await

async function doAsyncAwait(username){
     try{
        var result = await findByUsername(username);
        console.log(`User ${username} found with id ${result}`);
     }
     catch{
        console.log(`User ${username} not found - go ahead and create this user`);
     }
}

doAsyncAwait("jamiec")
doAsyncAwait("bob")

function doChain(username){
     findByUsername(username)
     .then( result =>  console.log(`User ${username} found with id ${result}`))
     .catch(() => console.log(`User ${username} not found - go ahead and create this user`))
}

doChain("jamiec");
doChain("bob");

    In both these cases you can return another promise and continue going with either async/await or chaining.

    const findByUsername = (name) => {
  return new Promise((res, rej) => {
    if(name === "jamiec"){
      res(1)
    }
    else{
       rej("No user found");
    }
  });
};

const createUser = (name) => {
   // simulate creating a user
   return new Promise(res => res(2));
}

// async await

async function doAsyncAwait(username){
     try{
        var result = await findByUsername(username);
        return result;
     }
     catch{
        return await createUser(name);
     }
}

(async function() {
  var id1 = await doAsyncAwait("jamiec")
  console.log(id1);
  var id2 = await doAsyncAwait("bob")
  console.log(id2);
})();

function doChain(username){
     return findByUsername(username)
      .catch(() => createUser(username))
}

doChain("jamiec").then(id => console.log(id));
doChain("bob").then(id => console.log(id));
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