Javascript fetch JSON with ES7 Async Await

es7-async-await.js

// Async/Await requirements: Latest Chrome/FF browser or Babel: https://babeljs.io/docs/plugins/transform-async-to-generator/
// Fetch requirements: Latest Chrome/FF browser or Github fetch polyfill: https://github.com/github/fetch

// async function
async function fetchAsync () {
  // await response of fetch call
  let response = await fetch('https://api.github.com');
  // only proceed once promise is resolved
  let data = await response.json();
  // only proceed once second promise is resolved
  return data;
}

// trigger async function
// log response or catch error of fetch promise
fetchAsync()
    .then(data => console.log(data))
    .catch(reason => console.log(reason.message))

@j2is from my understanding

1. await number 1(the inner one), used to await fetch() function
2. await number 2(the outter one), used to await res.json() function

Look at @kjkta answer might got you a clearer understanding in my humble opinion

Is that make sense?

Best,

if You have 2 fetch calls, its better to await the value as compared to the fetch calls themselves, because we will then let the process occur in parallel other than in sequence
like so

   async function bestFetch() {
     const first = fetch();
    const two = fetch();
    const firstvalue = await first.json();
    const secondvalue = await two.json();
  }

Unlike the following ( below ) where the requests will happen in sequence
The second resource will only be fetched once the first request has been resolved

async function badFetch() {
     const first = await  fetch();
    const two = await  fetch();
    const firstvalue = await first.json();
    const secondvalue = await two.json();}

thanks @Tevinthuku. The first example is awesome.

Example that can be easily tested with JSONPlaceholder :

(async () => {
  'use strict';
  console.clear();
  const getUser = async identifier => await (await fetch(`https://jsonplaceholder.typicode.com/users/${identifier}`)).json();
  try {
    const firstUser = await getUser(1);
    console.log(firstUser);
    const secondUser = await getUser(2);
    console.log(secondUser);
    // there are 10 users in JSONPlaceholder/Users
  } catch (exception) {
    console.error(`Failed to retrieve user informations: (${exception})`);
  }
})();

@gengns gets it.

You don't need to await the json. Just return it

async function fetchAsync () {
let response = await fetch('https://api.github.com');
return response.json();
}

Or this:

async function fetchAsync () {
return (await fetch('https://api.github.com')).json();
}

You should never return await, just return the promise itself return result.json() as @senner007 says.

This is a little confusing. In my testing if you call fetchAsync() from a non async method it doesn't wait. For example if the testFetch method below is called, the "after fetch()" is logged before the "data retrieved is logged". Am I missing something?? Please tell me I am!!! The only way I have seen it work as expected is when the fetchAsync method is called from a ngOnInit method that is also changed to async.

pubic testFetch() {
console.log('before fetch()');
fetchAsync();
console.log('after fetch()');
}

async function fetchAsync () {
// await response of fetch call
let response = await fetch('https://api.github.com');
// only proceed once promise is resolved
let data = await response.json();
// only proceed once second promise is resolved
console.log('data retrieved');
return data;
}

Ideally I want the json file loaded BEFORE execution continues like implied.

You should never return await, just return the promise itself return result.json() as @senner007 says.

That isn't very informative. Can you please explain why? I haven't directly used the async/await before. The fetch API documentation on MDN states that fetch returns a Promise, and also, that async implicitly returns a Promise

You should never return await, just return the promise itself return result.json() as @senner007 says.

That isn't very informative. Can you please explain why? I haven't directly used the async/await before...

Nevermind, I understand now. The fetch is async, the json method is sync. But in regards to that, I disagree with you comment that you should "never" return an await. If you wrap the fetch call in another function, you can abstract the call and have the calling code be responsible for accessing the response, may it be json, a blob, or something else. If, as you stated, you explicitly return the result by calling json(), that will fail unless the result is json.

@jagretz If you return await you are returning a promise anyway (e.g. you will still have to resolve that promise wherever you use the returned "value"). The issue with this is that you have still made the code synchronous (and therefore slower) for no gain.

More info: https://eslint.org/docs/rules/no-return-await

Pertinent quote:

Inside an async function, return await is seldom useful. Since the return value of an async function is always wrapped in Promise.resolve, return await doesn’t actually do anything except add extra time before the overarching Promise resolves or rejects. The only valid exception is if return await is used in a try/catch statement to catch errors from another Promise-based function.

Learned something new and useful today. Thanks for posting @atwright147 !

How would you change the script to put the output into a variable?

How would you change the script to put the output into a variable?

use a closure.

I came up to a similar use of fetch, but how are you dealing with errors?

async function async_fetch(url) {
  let response = await fetch(url)
  if (response.ok) return await response.json()
  throw new Error(response.status)
}

In addition to use try .... catch blocks, it is better to put the above code (by @gengns) inside the try block because "The Promise returned from fetch() won’t reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, it will resolve normally (with ok status set to false), and it will only reject on network failure or if anything prevented the request from completing." (quoted from https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch)

can u please tell me how to solve this

./src/useGoogleSearch.js
SyntaxError: /media/g/g-c/src/useGoogleSearch.js: Unterminated string constant (11:8)

9 | const fetchData = async () => {
10 | fetch(

11 | 'https://www.googleapis.com/customsearch/v1?key=$
| ^
12 | {API_KEY}&cx=${CONTEXT_KEY}&q=${term}'
13 | )
14 | .then(response => response.json())

@michaelnagy or use a fat arrow if you want to be fancy ;)

const fetchAsyncA = async () => 
	await (await fetch('https://api.github.com')).json()

As far as I can tell it's not possible to use the returned data in this case. Let me know if I'm missing something.

Even more fancy :)

import fetch from 'node-fetch'

export default async (...args) => await fetch(...args)

I want to execute a synchronous json fetch call. The following code (see below). It works in principle, but not in the expected order (not synchronously, but asynchronously again).
My log expectation is 1 2 3 4 5 6 7, but I get 1 2 3 7 4 5 6
How do I meet my expectation?

console.log("1 start");

async function fetchInfo() {
    let url = `https://reqres.in/api/products/3`; // only for test
    console.log("  3 fetch");
    let response = await fetch(url);
    console.log("  4 response");
    let data = await response.json()
    console.log("  5 return");
    return data;
}

console.log("2 call fetchInfo()")

fetchInfo()
    .then(data => console.log("6 response object: ...", data));

console.log("7 end / more code ...");

// Async/Await requirements: Latest Chrome/FF browser or Babel: https://babeljs.io/docs/plugins/transform-async-to-generator/
// Fetch requirements: Latest Chrome/FF browser or Github fetch polyfill: https://github.com/github/fetch

// async function
async function fetchAsync () {
// await response of fetch call
let response = await fetch('https://api.github.com');
// only proceed once promise is resolved
let data = await response.json();
// only proceed once second promise is resolved
return data;
}

// trigger async function
// log response or catch error of fetch promise