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"Synchronous" fetch with async/await

johnpaulada profile image John Paul Ada Updated on ・2 min read

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 lot – not 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);

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));

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);

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. 😄 😅

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();

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! 💙
Thanks! 👍

Posted on by:

johnpaulada profile

John Paul Ada

@johnpaulada

BA Psychology grad turned software engineer. Tech Evangelist. Data Scientist Wannabe.

Discussion

markdown guide
 

I find more readable :

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

Fetch returns a promise, no need to wrap this in an async function.

 

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

 

you need to wrap everything in async func

 

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?

 

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

 

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()

?

 

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.

 

Exactly what I needed to know too - thanks!

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

if fetch end in catch this Will Block the app.

 

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

 
 

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

})();
 
Sloan, the sloth mascot Comment marked as low quality/non-constructive by the community View code of conduct

Well i think that javacript is a s.hit because is for default async
and it's a nightmare is synchronizing processes. Fetch api and cors are more s.hit
they do not bring anything new, it's the same as ajax but with more pain.

 

That's because fetch is meant to be as bare bones as possible. Ajax is there to make fetch cleaner.

 
Sloan, the sloth mascot Comment marked as low quality/non-constructive by the community View code of conduct

await can only be used in async function. yes, it does wait for a promise, but function is still being run asynchronously. so when response is ready, the function to return this response has already ended.

so quit a pointless construction.