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Cover image for "Synchronous" fetch with async/await
John Paul Ada
John Paul Ada

Posted on • Updated on

"Synchronous" fetch with async/await

Originally posted at Medium.

TL;DR

async/await allows us to program using asynchronous requests in a "synchronous" manner using the modern versions of Javascript.

A hypothetical introduction

As web developers, we make requests to APIs a lotÒ€Š–Ò€Šnot only to our own APIs but to others’, too. As we all know, they can be a real pain in the ass.

What if we wanted to make a request to a hypothetical API https://api.com ?

What if I told you, you can make requests like this in Javascript?

const response = fetch('https://api.com/values/1');
const json = response.json();
console.log(json);
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Normally, this would be impossible, and normally you’d do something like this:

fetch('https://api.com/values/1')
    .then(response => response.json())
    .then(json => console.log(json));
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That is less readable than the former example.

Although, if you try the former example, it wouldn’t work. Why, you ask? Well folks, we’re missing the magic words!

await

await allows us to wait for the response of an asynchronous request.

To use awaitin our hypothetical code, we can do this:

const response = await fetch('https://api.com/values/1');
const json = await response.json();
console.log(json);
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Let’s break this down.

In the first line, we make a GET request to https://api.com/values/1. Instead of continuing to the next line, we wait for the request to finish, hence await. When it finishes, it passes the resolved value to the response variable.

In the second line, we get the JSON version of the response. Again, we use await so we can wait for it to complete (or fail) and then pass the result to the json variable.

Finally, in the last line, we log the value of the json variable to the console.

This saves us from writing less-than-adequately-readable code allows us to write cleaner code.

This is a more intuitive way of working with requests.

To help you understand it more, here’s another way of looking at this:

await allows us to wait for a Promise to resolve to a value.

await will return the value only after the Promise is resolved.

Sorry for being redundant but this is so you’d understand. Γ°ΕΈΛœβ€ž Γ°ΕΈΛœβ€¦

async

But wait, there’s more! Not really, but there’s something I haven’t told you yet. You know that code we rewrote with await? It won’t work yet.

WHAT!? Ikr.

To make it work, you need to wrap it inside an async function!

This is how you do it:

const request = async () => {
    const response = await fetch('https://api.com/values/1');
    const json = await response.json();
    console.log(json);
}

request();
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You just add the async keyword before the function declaration and run it! EZ!

But don’t just take it from me! Try it out here! Just hit the Run button:

async/await Runkit example.

Hit the heart, unicorn or hands up button below if you like this post! ðŸ’ℒ
Thanks! Γ°ΕΈβ€˜Β

Top comments (22)

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gtnovelt profile image
gt-novelt

I find more readable :

const json = await fetch('https://api.com/values/1')
                .then(response => response.json());
console.log(json);
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Fetch returns a promise, no need to wrap this in an async function.

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ewirth profile image
Ervin Wirth

Not working...
SyntaxError: await is only valid in async functions and async generators

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totalamd profile image
totalamd • Edited on

you need to wrap everything in async func

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uclaeamsavino profile image
uclaeamsavino • Edited on

Yo dog I heard you like await, so I put an await inside your await:

const data = await (await fetch('https://api.com/values/1')).json();

It would be nice if fetch offered a one-step method to get straight to the JSON data.

How do you get the nice sublime font colors?

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johnpaulada profile image
John Paul Ada Author

LOL
You could create a function for that.
I just create a javascript code block in markdown for the colors. :)

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archs profile image
Archs

your request function doesn't return the json result, only logs it. The function still returns a promise in my code, is it normal behavior? should the request function be sync when called as

const res = request()

?

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johnpaulada profile image
John Paul Ada Author • Edited on

ahh the function wrapping that should also be async, so it should be like:

const useRequest = async () => {
  const res = await request()
}

useRequest()

given that you return the json instead of just logging it in the request() function.

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mindyzwan profile image
Mindy Zwanziger

Exactly what I needed to know too - thanks!

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petko3000 profile image
petko3000

Guys, i would like to solve one issue with this async function. How to load output from that function to a variable usable in later non-async code? Like lets say i collect data from couple of APIs within async function, then i need to do some logic on top of that data with non-async code.

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spine001 profile image
spine001

You can pass an objetc to the function, and then take care to only change the content of the object without making a copy of it, then what is passed is the address of the object. And when you modify it with methods of the object that don't make copies, you will be modifying the object itself outside the scope of the function.
I imagine that you already solved this but since I haven't seen anybody ever give you an answer I am posting this to help others with the same problem.

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ahristoskov profile image
Alexander Hristoskov

I have the following problem, I have an express API endpoint which returns a token, although the promise .then() returns SyntaxError: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data like the Promise is still not resolved. Anyone else have such issues? I tried your code as well but with the same result.

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johnpaulada profile image
John Paul Ada Author

Hey! When I get this error, it's usually because the result I get isn't in JSON. Try await response.text() and check the response.

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venkatgadicherla profile image
venkatgadicherla

Hi
May be I am a bit naive. But this bit of code not working for me. I am trying to put this in a react component in componentDidMount to get data from asp.net controller and getting various errors. Any pre-requisites for this code to work.

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ben profile image
Ben Halpern

I've mostly stayed on the sidelines in terms of the new JavaScript features, but I'm starting to get back into it, and async await is something I look forward to playing with.

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stephendy profile image
Stephen πŸ‡¬πŸ‡§ πŸ‡ΊπŸ‡Έ πŸ‡ΈπŸ‡Ύ πŸ‡°πŸ‡΅

This is a great article, very new to this area and this is something that's been bugging me for days. I knew there had to be a better way to do what i wanted and this is it.

Plenty of examples out there going off on tangents or using jQuery etc - and also pointed me in the direction of something to read up on further.

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dann1609 profile image
dann1609

if fetch end in catch this Will Block the app.

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moustachedsign profile image
Moustache Design

Yes, indeed. You can prevent it by wrapping those lines in a try / catch block

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dimitir profile image
Dmitriy Popovich • Edited on

(async () => {
const todo = await Model.create(
{
//data
}
);
const todoItem = await todo.save();

})();
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mauroca80 profile image
mauroca80

great!!!!!!! thanks for all

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