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Javascript: How to access the return value of a Promise object

Katsiaryna (Kate) Lupachova on August 24, 2020

Originally posted on my personal blog. Intro (completely off-topic) It's has been almost 3 months since my last blog post. There are re...

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Dave Stewart • Jul 27 '22 • Edited

It's really important to note that the Promise object doesn't return a value, it resolves a value via the then() method.

It is the fetch() function that returns a value, which is a Promise instance.

It is the Promise instance on which you call the then() method, passing in a callback function, which will be eventually be fired when the async code finishes (and internally, calls resolve()).

As an example, here's what a custom load function might look like:

function load (url) {
  return new Promise(async function (resolve, reject) {
    // do async thing
    const res = await fetch(url)

    // your custom code
    console.log('Yay! Loaded:', url)

    // resolve
    resolve(res.json()) // see note below!
  })
}

// run the function and receive a Promise
const promise = load('https://...')

// let the Promise know what you want to do when it resolves
promise.then(console.log)

Note that the Promise object will resolve any nested promises as part of its work, so resolving res.json() which results in a Promise being created will be resolved internally before the final chained .then(console.log) is called.

The trick to Promises is:

always return Promises you create (or functions that return Promises) so they can be chained
always resolve() or reject() within the promise executor so .then() or .catch() will be called and the chaining will complete

uzoma • Jan 24 '23

Created an account just to say thank you for this breakdown.

Dave Stewart • Jan 24 '23

Well that's very kind of you. I'm glad it helped!

paint1024 • Apr 28 '21

Really COOL and helpful stuff!
It saves me from Promise hell.
I spent hours on Stack Overflow and Google and fixed my need with this great article.

Although I can only learn about Belarus through news and Lev Vygotsky, I hope your country makes it.
Thank you again.
Best wishes from Taiwan.

Katsiaryna (Kate) Lupachova • Apr 29 '21

Really happy that my blog post was helpful to you!

And thanks for your support! We (Belarusians) must win this fight!

Alexey Zyukov • Aug 22 '21

Thank for the explanation of code.
...when you win you'll understand that you loose, but too late. from Ukraine with best wishes.

Irina Kats • Oct 5 '23

I am another one who created an account to say Thank You to Kate for the article and @davestewart for amplifying it! I've been having this trouble too and wanted to understand what I am doing wrong. Now I seem to understand it.

TomKnn • Jun 5 '22

Hi Kate,

Thanks for the article.
I only get results when I write down the code (with my own api endpoint) like this:
fetch("https://api.spoonacular.com/recipes/complexSearch?number=2&apiKey=b4408aa9ab144e47ae2bf8eff93e72f5")
.then((response) => response.json())
.then((user) => {
console.log(user)
});

I don`t understand what you mean by console.log(user.address) or how to adapt this into your own code.

Anyway, from the results:
results: Array(2)
0: {id: 716426, title: 'Cauliflower, Brown Rice, and Vegetable Fried Rice', image: 'spoonacular.com/recipeImages/71642...', imageType: 'jpg'}
1: {id: 715594, title: 'Homemade Garlic and Basil French Fries', image: 'spoonacular.com/recipeImages/71559...', imageType: 'jpg'}
length: 2

I want to make an array with only the recipe numbers, something like this:
recipeArray = [716426, 715594] and use this for a declaration like let defaultIndex = recipeArray[0];- for another piece of code.

Do you know how to do this? I`d be very grateful.

Katsiaryna (Kate) Lupachova • Jun 11 '22

You can try something like that:

const getRecipes = async () => {
fetch("https://api.spoonacular.com/recipes/complexSearch?number=2&apiKey=b4408aa9ab144e47ae2bf8eff93e72f5")
.then((response) => response.json())
.then((user) => {
const recipes = user.results.map(result => result.id)
console.log(recipes)
return recipes
});
}

const recipes = await getRecipes();

You'll get an array of recipe ids.

TomKnn • Jun 13 '22

Hi Kate,

Thank you so much for your time and trouble, I appreciate!
I've tried your code but I get this error because of the 'const recipes = await getRecipes()' line:
Uncaught SyntaxError: await is only valid in async functions and the top level bodies of modules

Do you have any suggestions? Api fetch is pretty new to me and my vocabulary with this is limited.

Katsiaryna (Kate) Lupachova • Jun 13 '22

could you please DM me or provide me a link to your code somewhere online (github repo, code sanbox and so on), so I could take a look and try to help you?

TomKnn • Jun 14 '22

Sure, it`s github.com/TomKnn/EindopdrachtMood....
To give you some context, I'm re-educating from illustrator/'art'teacher to front end developer due to lack of work opportunities. I like it very much but the course is online with little help, I'm stuck for months now on little things.
This is my final assignment where we have to a modest site design with an api function.
It's a bit of a mess, but I'll tweak it as soon as my code works.
The files you need are here github.com/TomKnn/EindopdrachtMood...
And here is the main.js where I want to import the function.
github.com/TomKnn/EindopdrachtMood...

Pls let me know if something is unclear, many thanks!!
Tom

gloriaconcepto • Dec 12 '21

First of all i guess you are feeling ok,secondly i send my positive thoughts of love peace to your country.Lastly thanks i was in a hell trap .but your articles has really help to sort it out one Love .

Katsiaryna (Kate) Lupachova • Dec 13 '21

Thank you very much! Appreciate your words!

Monotheist • Mar 16 '21

const address = fetch("jsonplaceholder.typicode.com/users/1")
.then((response) => response.json())
.then((user) => {
return user.address;
});

const printAddress = async () => {
const a = await address;
console.log(a);
};

let d = printAddress();
console.log(d); <-------- how do I do this? this still returns a Promise { }
I want to access return value from printAddress()

Katsiaryna (Kate) Lupachova • Mar 18 '21

That's the whole point! You can use the return value from the Promise object ONLY in a async function.

You should do all the stuff that you'd like to do with the return result in a printAddress() async function.

thechallengerr • Oct 26 '22 • Edited

But what if i had to take the return value of Promise to use as data to render ?
my case is that : I have to query database to get the link for 'src' attribute ? I use mongoosejs
Hope you can explain . Thank You

async getEventThumb(event_slug) {
            let event_thumb = Event.findOne({ event_slug: event_slug }).then((event) => {
                if (event === null) {
                    return ""
                } else {
                    event_thumb = event.event_thumb;
                }

                return event.event_thumb;
            }).catch((err) => {
                console.error(err);
            });

            // console.log(event_thumb);

            return await event_thumb;
        }

younglotushuncho • Jan 13 '22

i find a new solution

the best way to use a promise ...... use a promise inside a promise ...it will work

small-basic • Oct 5 '21

how comes you are invoking the "address" function without any parenthesis.

const printAddress = async () => {
const a = await address;
console.log(a);
};

total noob question here i guess, but would appreciate if someone can direct me to the right resource to learn this further , tia

Katsiaryna (Kate) Lupachova • Oct 8 '21

because "address" in not a function here, but a variable to which the return value of the fetch request is assigned to

small-basic • Oct 21 '21

thanks for the reply.

Just one last question, if the "address" is a variable which stores the result of the fetch query, then what is triggering the fetch call ?.

Katsiaryna (Kate) Lupachova • Oct 22 '21

once the interpreter ((like the JavaScript interpreter in a web browser) ) reaches the code with the fetch() function, it adds it to the call stack list. We just assign the result (the return value) of this async function in order to have access to it later.

The call stack order is really easy to test. If we update the code like this:

fetch("https://jsonplaceholder.typicode.com/users/1")
  .then((response) => response.json())
  .then((user) => {
    console.log("FETCH RETURN");
    return user.address;
  });

const printAddress = async () => {
  console.log("PRINT ADDRESS");
//   const a = await address;
//   console.log(a);
};

console.log("RUN");
printAddress();

The result in the console will be:
RUN
PRINT ADDRESS
FETCH RETURN

The fetch function is being executed without any additional trigger or call. But we get its result after all other functions because fetch takes more time to complete its job. And as fetch is an asynchronous function, it's not blocking other functions.

small-basic • Oct 22 '21

thank you so much

Artem • May 24 '21

Well, that is not exactly a solution as far as I can understand the problem (or confusion if you will) that was supposed to be solved by this article.

In the first part, you are showing an assignment to a var "address" and then you are trying to log it, which returns a promise as was expected, right?

Then you are proposing to add another function and call "address" from the inside, but you DO NOT log "printAddress", you invoke it. You got logged data, but the same result could have been achieved by just logging the response in "address".

As for "printAddress", by making in async function you have converted it to a promise, so by trying to log it, you will get the same result as logging "address".

Correct me if I'm mistaken, but it seems to me your solution doesn't solve anything but only making the code bigger. The response date was logged from inside the promise and hasn't been extracted in any way.

The problem that you have tried to solve is definitely bothering a number of junior devs (me also as one of those juniors), however, I see no solution here. Moreover, I am, frankly, displeased to see unchecked, untrustworthy content here, no offense.

Katsiaryna (Kate) Lupachova • Jun 1 '21 • Edited

The blog post consists of two parts:

The first part describes how to access the return value of a Promise object right after it has been resolved
The second part describes two possible ways of accessing the return value of Promise object LATER in the code. It's a completely another use case. Yes, the result is the same as when logging "address" variable (as it should be). But the main idea is to describe how to access this address value LATER in the code.

Probably you have another use case, that's why this solution doesn't meet your needs. If you could share the problem that you are trying to solve, maybe we could discuss the possible solution.

Johnny Rubi • Dec 24 '21

Hello friend, I'm a Brazilian guy who spent 10 hours trying to understand why I couldn't get the result from my API, I spent hours trying to find an answer on the internet on all possible pages and I didn't understand or found any answer, I ended up on your blog and wanted to thank you , I signed up on this site just to comment that your job saved me in every way possible, as I needed it to get a job! please don't stop your blog! you are awesome! and teaches very well! thanks!

Katsiaryna (Kate) Lupachova • Dec 24 '21

Hi Johnny! Thank you for such nice words! It means a lot to me!
The main reason why I write blog posts is to share my knowledge with fellow developers so we could help each other.

Augusto Martinez • Aug 15 '21

This brought light to an issue I had after being hours looking over the internet, I am just curios that on my implementation (using Vue.js), the first solution works only when arrow functions are used, if I try to convert the arrow function to a regular function, the promise value is never returned.

Kaustubh Dwivedi • Jan 28 '22

const address = fetch("jsonplaceholder.typicode.com/users/1")
.then((response) => response.json())
.then((user) => {
return user.address;
});

const printAddress = async () => {
const a = await address;
return a;
};

*Console.log(a) inside printAddress is working fine but :
Now I need to call this printAddress inside the return of a react Component thats why I am returning 'a' from printAdress, there it is showing promise pending. Is there any way to solve this ? *

Katsiaryna (Kate) Lupachova • Feb 1 '22

could you provide a piece of code of your react component? it would be easier to understand and will try to help.

stefcab • Jan 4

You could even simplify without using an other function:

const printAddress = () => {
  address.then((a) => {
    console.log(a);
  });
};

address.then((a) => {
    console.log(a);
  });

Stefan Wolfrum • Jan 27 '22

Great article, helped me a lot to understand this concept! 👍👍
One question: suppose this fetch() call is inside a function. How would I return the JSON object from that function back to the main JS area where I made the call to the function? I tried a "return a;" just after the "console.log(a);" but that didn't work. Then I tried "return printAddress();", which didn't work either.
Any help? 😳

Katsiaryna (Kate) Lupachova • Feb 1 '22

I'd say that the only way to get this JSON object outside the fetch() call is to assign it to a variable. If you could provide your piece a code it would be easier to understand and to help:)

rishimagiawala • Dec 17 '20

Thank you so much! You have no idea how helpful this simple guide was for me. I really mean it. Thank you for taking the time to write this.

Katsiaryna (Kate) Lupachova • Dec 19 '20

Thank you for such nice words! Happy to help!

Katsiaryna (Kate) Lupachova • May 5 '21

Thank you for your nice words!
Glad to know that my blog post helped you with your project!

Sadaf Sajad • Jul 14 '22

Hi, Kate
Can you please help me with this one
i receive data from an axios call collect it in a variable and return it when i call the function i receive data as promise like you showed
i put async await everything you said but this isn't working

$(document).ready(function () {
merchantDetails;
var load = merchantDetails(userName)
console.log(load)
})
merchantDetails = function (userName) {
var axiosResponse = axios.get("/adminMerchantOrderDetails/merchantDetails?username=" + userName).then(function (response) {
let order = response.data.responseData;
return order
console.log((order))
})
return axiosResponse
}

Katsiaryna (Kate) Lupachova • Jul 17 '22

Hi Sadaf! It's kinda hard to understand this unformatted piece of code. Could you please provide a link to a codesanbox, GitHub repo or something like that so I could take a look and try to help you?

John Balladares • Jun 9 '21

Thank you so much for this!!

Buraq Abdul Kareem • Feb 21 '21

Thanks Kate, I've just start web applications coding and I wonder how can I manipulate the data instead of print it out only.

Thanks x

fs10218 • Oct 13 '21 • Edited

Thank you, this article was very helpful;
But you did not say how to call the Promise value elsewhere and just print it on the console
Wishing you victory and success
from Iran