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Learning Promises, Async/Await to control execution order

I have been studying promises, await and async functions. While I was just in the stage of learning promises, I realized that the following: When I would send out two requests, there was no guarantee that they would come in the order that they are written in the code. Of course, with routing and packets of a network. When I ran the code below, the requests would resolve in no specific order.
const getCountry = async country => {
await fetch(`https://restcountries.com/v2/name/${country}`)
.then(res => res.json())
.then(data => {
console.log(data[0]);
})
.catch(err => err.message);
};
getCountry('portugal');
getCountry('ecuador');
At this point, I hadn't learned about async and await. So, the following code works the exact way I want it. Each request, waits until the other one is done.
Is this the most simple way to do it? Are there any redundancies that I could remove? I don't need a ton of alternate examples; unless I am doing something wrong.
await fetch(`https://restcountries.com/v2/name/${country}`)
.then(res => res.json())
.then(data => {
console.log(data[0]);
})
.catch(err => err.message);
};
const getCountryData = async function () {
await getCountry('portugal');
await getCountry('ecuador');
};
getCountryData();
Thanks in advance,

Yes, that's the correct way to do so. Do realize though that you're blocking each request so they run one at a time, causing inefficiency. As I mentioned, the beauty of JavaScript is its asynchronism, so take advantage of it. You can run all the requests almost concurrently, causing your requests to speed up drastically. Take this example:
// get results...
const getCountry = async country => {
const res = await fetch(`https://restcountries.com/v2/name/${country}`);
const json = res.json();
return json;
};
const getCountryData = async countries => {
const proms = countries.map(getCountry); // create an array of promises
const res = await Promise.all(proms); // wait for all promises to complete
// get the first value from the returned array
return res.map(r => r[0]);
};
// demo:
getCountryData(['portugal', 'ecuador']).then(console.log);
// it orders by the countries you ordered
getCountryData(['ecuador', 'portugal']).then(console.log);
// get lots of countries with speed
getCountryData(['mexico', 'china', 'france', 'germany', 'ecaudor']).then(console.log);
Edit: I just realized that Promise.all auto-orders the promises for you, so no need to add an extra sort function. Here's the sort fn anyways for reference if you take a different appoach:
myArr.sort((a, b) =>
(countries.indexOf(a.name.toLowerCase()) > countries.indexOf(b.name.toLowerCase())) ? 1 :
(countries.indexOf(a.name.toLowerCase()) < countries.indexOf(b.name.toLowerCase()))) ? -1 :
0
);

I tried it the way #deceze recommended and it works fine: I removed all of the .then and replaced them with await. A lot cleaner this way. Now I can use normal try and catch blocks.
// GET COUNTRIES IN ORDER
const getCountry = async country => {
try {
const status = await fetch(`https://restcountries.com/v2/name/${country}`);
const data = await status.json();
renderCountry(data[0]); // Data is here. Now Render HTML
} catch (err) {
console.log(err.name, err.message);
}
};
const getCountryData = async function () {
await getCountry('portugal');
await getCountry('Ecuador');
};
btn.addEventListener('click', function () {
getCountryData();
});
Thank you all.

How do I access the value returned by promise firestore?

I'm trying to access what's returned by secondSnapshot.data(), but am having an issue, as described by the comments below. I tried to create an async function, but to no avail. Any idea what's going wrong? Please view the 2 comments.
useEffect(() => {
firestore.collection(`comments`).onSnapshot((snapshot) => {
const posts = snapshot.docs
.map((doc) => {
const address = doc.data().comments?.map((comment) => {
comment.get().then((secondSnapshot) => {
console.log("snapshot", secondSnapshot.data());
#I SEE WHAT I EXPECT TO SEE
return secondSnapshot.data();
});
});
console.log(address) #THIS RETURNS UNDEFINED FOR SOME REASON??
return {
username: doc.data().username,
date: doc.data().date.seconds,
text: doc.data().text,
votes: doc.data().votes,
comments: [],
};
});
props.setComments(posts);
});
}, [location]);

Besides the React vs Firebase design problem pointed by #Mulan, your code has several issues.
Issue #1
The following map function returns nothing. You won't find anything in address if you don't return something from this block.
const address = doc.data().comments?.map((comment) => {
/* This MUST return something */
});
Issue #2
You are mixing synchronous and asynchronous code. Basically, your attempt to print address value before the different async calls to comment.get() finished running.
const address = doc.data().comments?.map((comment) => {
/* async code inside */
});
console.log(address); // this runs without waiting for the async code
Advice
If you are having a hard time with the old promise syntax, try using async / await instead:
firestore.collection(`comments`).onSnapshot(async (snapshot) => {
const postPromises = snapshot.docs
.map(async (doc) => {
const comments = doc.data().comments ?? []; // assign an empty array if there are no comments
const secondSnapshots = await Promise.all(comments.map((comment) => comment.get()));
const addresses = secondSnapshots.map((secondSnapshot) => secondSnapshot.data());
console.log(addresses);
return { /* ... */ };
});
const posts = await Promise.all(postPromises);
props.setComments(posts);
});
I'm not sure you should be calling comment.get() inside onSnapshot, but this is another story and I'm not a Firebase specialist.

Defining asynchrosnous function and using it in a forEach loop

I am trying to compile tex files into PFD using data from a firestore database. After completion the script doens't terminate. I found one can use process.exit() to make it quit. However, it terminates the child processes still cimpling the tex files. So, I need an asynchronous function.
The guides I found on how to make them did not particularly help me. I am still very new to javascript and any bloat is still highly confusion to me.
The guides I found on how to make them did not particularly help me. I am still very new to javascript and any bloat is still highly confusion to me.
prepending below mentioned makePDF function with async and the call of the function with await does not work and is, to my understanding, outdated.
I tried implementing a promise, but don't understand how their syntax works. Does simply appending .then() to the function call in the for loop suffice to make the loop wait for the functions completion?
How do I make this specific asynchronous?
Does it matter that it already uses asynchronous functions in its body?
I understand that a promise is used to return what ever the producer has produced to a consumer. Now, my producer doesn't produce anything to be returned. Does this matter at all?
My function called from the for loop:
function makePDF(object){
let input = fs.readFileSync('main.tex', 'utf8');
const outNameTex = object.ID + '.tex';
const outNamePDF = object.ID + '.pdf';
makeTEX(object, input, outNameTex);
const infile = fs.createReadStream(outNameTex);
const outfile = fs.createWriteStream(outNamePDF);
const pdf = latex(infile);
pdf.pipe(outfile);
pdf.on('error', err => console.error(err));
pdf.on('finish', () => {console.log('PDF generated!')});
}
And my function with the loop:
firebase.auth().onAuthStateChanged((user) => {
if (user) {
console.log('user');
db.collection('objects').where('printed', '==', false).get().then((snapshot) => {
snapshot.forEach((doc) => {
console.table(doc.data());
makePDF(doc.data());
})
process.exit();
})
.catch((err) => {
console.log('Error getting documents', err);
});
} else {
console.log('no user');
}
});
It outputs a table for each document, but no PDF generated.

async/await can be tricky to use with for loops, that is because async functions return a promise... if you convert the async/await syntax to native promise syntax you might figure out what the issue is.
What you want to do is use Array.map to map/convert each doc to a promise that resolves once the makePDF is done, then use Promise.all to wait for all the promises to resolve..
The solution should look something like this
function makePDF(object){
return new Promise((resolve, reject) => {
let input = fs.readFileSync('main.tex', 'utf8');
const outNameTex = object.ID + '.tex';
const outNamePDF = object.ID + '.pdf';
makeTEX(object, input, outNameTex);
const infile = fs.createReadStream(outNameTex);
const outfile = fs.createWriteStream(outNamePDF);
const pdf = latex(infile);
pdf.pipe(outfile);
pdf.on('error', reject);
pdf.on('finish', () => {console.log('PDF generated!'); resolve();});
}
firebase.auth().onAuthStateChanged((user) => {
if (user) {
console.log('user');
db.collection('objects').where('printed', '==', false).get().then((snapshot) => {
const promiseArr = snapshot.docs.map((doc) => {
console.table(doc.data());
return makePDF(doc.data());
})
Promise.all(promiseArr)
.then(() => {
process.exit();
})
})
.catch((err) => {
console.log('Error getting documents', err);
});
} else {
console.log('no user');
}
});

Why can't I move "await" to other parts of my code?

Edit2: Solution at the bottom
I am using the chrome-console and I am trying to output fetched data, and I only get the desired output by writing "await" at exactly the right place, even though another solution can do it earlier and I don't know why/how it works.
solution() is the "official" solution from a web-course I am doing. Both functions return the same, currently. In myFunction I tried writing "await" in front of every used function and make every function "async", but I still can't replace the "await" inside log, even though the other solution can.
const urls = ['https://jsonplaceholder.typicode.com/users']
const myFunction = async function() {
// tried await before urls/fetch (+ make it async)
const arrfetched = urls.map( url => fetch(url) );
const [ users ] = arrfetched.map( async fetched => { //tried await in front of arrfetched
return (await fetched).json(); //tried await right after return
});
console.log('users', await users); // but can't get rid of this await
}
const solution = async function() {
const [ users ] = await Promise.all(urls.map(async function(url) {
const response = await fetch(url);
return response.json();
}));
console.log('users', users); // none here, so it can be done
}
solution();
myFunction();
I would think "await" works in a way that makes:
const a = await b;
console.log(a); // this doesn't work
the same as
const a = b;
console.log(await a); // this works
but it doesn't, and I don't understand why not. I feel like Promise.all does something unexpected, as simply writing "await" in the declaration can't do the same, only after the declaration.
Edit1: this does not work
const myFunction = async function() {
const arrfetched = await urls.map( async url => await fetch(url) );
const [ users ] = await arrfetched.map( async fetched => {
return await (await fetched).json();
});
console.log('users', users);
}
Edit2: Thanks for the help everyone, I tried putting ".toString()" on a lot of variables and switching where I put "await" in the code and where not.
As far as I understand it, if I don't use Promise.all then I need to await every time I want to use (as in the actualy data, not just use) a function or variable that has promises. It is insufficient to only have await where the data is being procensed and not further up.
In the Edit1 above users runs bevore any other await is complete, therefore no matter how many awaits i write in, none are being executed. Copying this code in the (in my case chrome-)console demostrates it nicely:
const urls = [
'https://jsonplaceholder.typicode.com/users',
]
const myFunction = async function() {
const arrfetched = urls.map( async url => fetch(url) );
const [ users ] = arrfetched.map( async fetched => {
console.log('fetched', fetched);
console.log('fetched wait', await fetched);
return (await fetched).json();
});
console.log('users', users);
console.log('users wait', await users);
}
myFunction();
// Output in the order below:
// fetched()
// users()
// fetched wait()
// users wait()

TL; DR: Promise.all is important there, but it's nothing magical. It just converts an array of Promises into a Promise that resolves with an array.
Let's break down myFunction:
const arrfetched = urls.map( url => fetch(url) );
This returns an array of Promises, all good so far.
const [ users] = arrfetched.map( async fetched => {
return (await fetched).json();
});
You're destructuring an array to get the first member, so it's the same as this:
const arr = arrfetched.map( async fetched => {
return (await fetched).json();
});
const users = arr[0];
Here we are transforming an array of promises into another array of promises. Notice that calling map with an async function will always result in an array of Promises.
You then move the first member of that array into users, so users now actually contains a single Promise. You then await it before printing it:
console.log('users', await users);
In contrast, the other snippet does something slightly different here:
const [ users ] = await Promise.all(urls.map(async function(url) {
const response = await fetch(url);
return response.json();
}));
Once again, let's separate the destructuring:
const arr = await Promise.all(urls.map(async function(url) {
const response = await fetch(url);
return response.json();
}));
const users = arr[0];
Promise.all transforms the array of Promises into a single Promise that results in an array. This means that, after await Promise.all, everything in arr has been awaited (you can sort of imagine await Promise.all like a loop that awaits everything in the array). This means that arr is just a normal array (not an array of Promises) and thus users is already awaited, or rather, it was never a Promise in the first place, and thus you don't need to await it.

Maybe the easiest way to explain this is to break down what each step achieves:
const urls = ['https://jsonplaceholder.typicode.com/users']
async function myFunction() {
// You can definitely use `map` to `fetch` the urls
// but remember that `fetch` is a method that returns a promise
// so you'll just be left with an array filled with promises that
// are waiting to be resolved.
const arrfetched = urls.map(url => fetch(url));
// `Promise.all` is the most convenient way to wait til everything's resolved
// and it _also_ returns a promise. We can use `await` to wait for that
// to complete.
const responses = await Promise.all(arrfetched);
// We now have an array of resolved promises, and we can, again, use `map`
// to iterate over them to return JSON. `json()` _also_ returns a promise
// so again you'll be left with an array of unresolved promises...
const userData = responses.map(fetched => fetched.json());
//...so we wait for those too, and destructure out the first array element
const [users] = await Promise.all(userData);
//... et voila!
console.log(users);
}
myFunction();

Await can only be used in an async function. Await is a reserved key. You can't wait for something if it isn't async. That's why it works in a console.log but not in the global scope.

basic async await javascript

I've got this very basic async await which I believe is not running correctly:
const ex = {
people: () => {console.log('people')},
places: () => {console.log('places')}
}
async function example() {
try {
const people = await ex.people()
const places = await ex.places()
} catch(err) {
console.log(err)
}
}
example().then(console.log('last'))
output:
people
last
places
Shouldn't this be outputting 'people, places, last'?

Yes, this code snippet is pretty much ok!
The only thing missing here it's that promises returns functions.
So .then should be like this:
example().then(() => console.log('last'))
Hope it works for you!
Edit:
As #gyre said, ex.people and ex.places should return promises in order to work correctly
Here's an example