Return from a promise then()

Question

I have got a javascript code like this:

function justTesting() {
  promise.then(function(output) {
    return output + 1;
  });
}

var test = justTesting();

I have got always an undefined value for the var test. I think that it is because the promises are not resolved yet..there is a way to return a value from a promise?

Answer 1

When you return something from a then() callback, it's a bit magic. If you return a value, the next then() is called with that value. However, if you return something promise-like, the next then() waits on it, and is only called when that promise settles (succeeds/fails).

Source: https://web.dev/promises/#queuing-asynchronous-actions

Answer 2

To use a promise, you have to either call a function that creates a promise or you have to create one yourself. You don't really describe what problem you're really trying to solve, but here's how you would create a promise yourself:

function justTesting(input) {
    return new Promise(function(resolve, reject) {
        // some async operation here
        setTimeout(function() {
            // resolve the promise with some value
            resolve(input + 10);
        }, 500);
    });
}

justTesting(29).then(function(val) {
   // you access the value from the promise here
   log(val);
});

// display output in snippet
function log(x) {
    document.write(x);
}

Or, if you already have a function that returns a promise, you can use that function and return its promise:

// function that returns a promise
function delay(t) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve();
    }, t);
  });
}

function justTesting(input) {
  return delay(100).then(function() {
    return input + 10;
  });
}

justTesting(29).then(function(val) {
  // you access the value from the promise here
  log(val);
});

// display output in snippet
function log(x) {
  document.write(x);
}

Answer 3

What I have done here is that I have returned a promise from the justTesting function. You can then get the result when the function is resolved.

// new answer

function justTesting() {
  return new Promise((resolve, reject) => {
    if (true) {
      return resolve("testing");
    } else {
      return reject("promise failed");
   }
 });
}

justTesting()
  .then(res => {
     let test = res;
     // do something with the output :)
  })
  .catch(err => {
    console.log(err);
  });

Hope this helps!

// old answer

function justTesting() {
  return promise.then(function(output) {
    return output + 1;
  });
}

justTesting().then((res) => {
     var test = res;
    // do something with the output :)
    }

Answer 4

I prefer to use "await" command and async functions to get rid of confusions of promises,

In this case I would write an asynchronous function first, this will be used instead of the anonymous function called under "promise.then" part of this question :

async function SubFunction(output){

   // Call to database , returns a promise, like an Ajax call etc :

   const response = await axios.get( GetApiHost() + '/api/some_endpoint')

   // Return :
   return response;

}

and then I would call this function from main function :

async function justTesting() {
   const lv_result = await SubFunction(output);

   return lv_result + 1;
}

Noting that I returned both main function and sub function to async functions here.

Answer 5

Promises don't "return" values, they pass them to a callback (which you supply with .then()).

It's probably trying to say that you're supposed to do resolve(someObject); inside the promise implementation.

Then in your then code you can reference someObject to do what you want.

Answer 6

You need to make use of reference data type like array or object.

function foo(u,n){
  let result = [];
  const userBrands = new Promise((res, rej)=> {
                        res(['brand 1', 'brand 3']);
                      })
  
  userBrands.then((ub)=>{
    return new Promise((res, rej) =>{
      res([...ub, 'brand 4', 'brand 5']);
    })
  }).then(response => {
    return result.push(...response);
  });
  return result;
};
foo();

Answer 7

You cannot return value after resolving promise. Instead call another function when promise is resolved:

function justTesting() {
    promise.then(function(output) {
        // instead of return call another function
        afterResolve(output + 1);
    });
}

function afterResolve(result) {
    // do something with result
}

var test = justTesting();