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Promises: combine single resolve methods and Promise.all()

I have multiple promises fetching different assets. I want to achieve:
Executing some code first to handle some of these promises via then().
When everything is resolved/fetched, executing some code that launches my app via Promise.all().then().
I need to make sure 1 is excuted before 2. It does seem to work from what I have tested.
// handle promises one by one
promise1.then(results => {});
promise2.then(results => {});
// common process that should come after the above then()s
Promise.all([promise1, promise2])
.then(results => {});
But can I rely on it? Are "single" then() always executed before a then() on Promise.all()?

While Promise.all is stalled because it waits for all the promises to finish, there is no guarantee that it will resolve after the .then call on the promise that resolves last.
Instead you should try creating new promises from the .then calls.
// handle promises one by one
const promise3 = promise1.then(results => {});
const promise4 = promise2.then(results => {});
// common process that should come after the above then()s
Promise.all([promise3, promise4])
.then(results => {});
This way you can guarantee that the Promise all will resolve after the then calls

You can around by using Async/await. With waiting for each promise, you can write code to process data at the end.
Example:
(async () => {
const responses = [];
await new Promise((res) => res("promise1")).then((res) => {
console.log("From promise1 first then");
responses.push(res);
});
// Code at the end to do after all promises
console.log("Coming from second");
console.log("Responses:", responses);
})();

Should I use await inside Promise.all?

I'm building express middleware to make two async calls to the database to check if the username or email are already in use. The functions return promises without a catch, as I want to keep database logic seperate from req/res/next logic, and I have centralised error handling that requires next as an argument. In my postman testing on local environment the following code works as expected and my centralised errorhandler returns the error to the client:
async checkUsernameExists(username) {
await this.sequelize.transaction(
async () =>
await this.User.findOne({
where: {
username,
},
}).then(user => {
if (user) throw new Conflict('Failed. Username already in use.');
}),
);
}
const checkDuplicateUsernameOrEmail = async (
{ body: { email, username } },
res,
next,
) => {
await Promise.all([
checkUsernameExists(username),
checkEmailExists(email),
])
.then(() => next())
.catch(error => next(error));
};
However, as the checkExists functions are async, shouldn't they be included in Promise.all with await? Or does Promise.all do this automatically?
await Promise.all([
await checkUsernameExists(username),
await checkEmailExists(email),
])...
This leads to an unhandled promise rejection from checkUsernameExists and no response is sent back to the client.

Should I use await inside Promise.all?
No (at least not the way you're doing it). Promise.all accepts and expects an array of Promises. Once they all resolve, or if one rejects, the Promise.all will resolve or reject. If you use await, you'll be passing an array of plain non-Promise values to Promise.all, which isn't the logic you want. If you use await, you'll also be waiting for the Promises to resolve serially, rather than in parallel, defeating the entire point of Promise.all. For example:
await Promise.all([
await checkUsernameExists(username),
await checkEmailExists(email),
])...
If checkUsernameExists takes 0.5 seconds to resolve, and checkEmailExists also takes 0.5 seconds to resolve, it will take at least 1 second for the Promise.all to resolve, because the Promises are being resolved via the await checkUsernameExistss, not by the Promise.all itself.
You should definitely do
await Promise.all([
checkUsernameExists(username),
checkEmailExists(email),
])
Async functions return Promises - to the Promise.all, someFnThatReturnsAPromise() is the same as somePromise. So there's absolutely nothing wrong with invoking the function and putting the resulting Promise in the array to pass to Promise.all.

Node: Using promise.all() to call API's in parallel?

I'm a bit confused how promise.all work, does it run the array of promises in parallel?
So here is a sample code
// index.js
const getSomething = async (args) => {
return await apiCallHere(args)
}
// Create Array of Promises
const arrayOfPromises = sampleArray.map(sample => new Promise((resolve, reject) => {
try {
const something = this.getSomething(sample, args)
resolve(something)
} catch (error) {
reject(error)
}
}))
await Promise.all(arrayOfPromises)
From what I observed, Promise.all runs the promises in parallel, and wait for all promises
to finish.

does it run the array of promises in parallel
Promise.all doesn't, no; your code does (well, probably; see the Notes below). The work is already underway before Promise.all sees the promises. What Promise.all does is give you a promise that will settle when all of the promises you give it are fulfilled (or one of them is rejected).
It's your code that makes the work run in parallel, by starting the actions that the promises report the completion of (in the map callback) in order to give them to Promise.all in the first place. See *** comments:
// *** `map` is synchronous, it loops all the way through the array
const arrayOfPromises = sampleArray.map(sample => new Promise((resolve, reject) => {
try {
const something = this.getSomething(sample, args) // *** This is what starts each thing
resolve(something)
} catch (error) {
reject(error)
}
}))
// *** The work is already underway here
// *** This just waits for it to finish
await Promise.all(arrayOfPromises)
Remember that a promise is just a way to observe the completion of an asynchronous process. Promises don't run anything. They just report the completion of something, along with the fulfillment value or rejection reason.
Notes
If this.getSomething(sample, args) returns a promise, your code is falling prey to the explicit promise creation anti-pattern: There's no reason to use new Promise here at all. Instead:
const arrayOfPromises = sampleArray.map(sample => this.getSomething(sample, args));
If this.getSomething(sample, args) returns its value immediately, then there's no point in using promises here at all, because the operations are already complete by the time it returns.
(I assume it doesn't start an asynchronous process and report completion via a callback instead of a promise, since you haven't shown a callback but you have shown using the return value.)
The getSomething you've shown in the question returns a promise (because it's an async function), but you wouldn't call that as this.getSomething(...), just as getSomething(...).

Make Promise.race finish only when one is successful

I have three asynchronous functions that return an object. If one of the functions are not successful, they return an empty object. The issue is that if I try to Promise.race these functions, one of the not successful functions may finish first and have an empty object return. My Promise code looks like this:
let product = await Promise.race([
get_info_1(code),
get_info_2(code),
get_info_3(code)
])
Is it possible to stop one of the asynchronous get_info functions from running without stopping the Promise.race? Or could I somehow tell Promise.race to wait until the returned object contains some information?

You can make the rejected Promise never complete
const never_resolve = new Promise(() => {})
{
(async function() {
let product = await Promise.race([
Promise.reject(1).catch(_ => never_resolve),
Promise.resolve(2).catch(_ => never_resolve),
Promise.reject(3).catch(_ => never_resolve)
])
console.log(product)
})()
}

How do I get data from pending resolved promise without async/await?

I have abstraction:
function fetchDataFromAPI() {
const url = `https://api...`
return fetch(url).then(response => response.json())
}
I want to use it in my other piece of code like:
if(something){
const data = fetchDataFromAPI()
return data
}
if I console.log data what I get is resolved pending promise
Promise {<pending>}
__proto__: Promise
[[PromiseStatus]]: "resolved"
[[PromiseValue]]: Object
How do I get that Object in data instead of Promise?

You can not. Here is why:
Promise is a language construct that makes JavaScript engine to continue to execute the code without waiting the return of inner function, also known as the executor function. A promise always run inside the event loop.
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
console.log(p);
Basically a promise is a glorified syntactic sugar for a callback. We will see how but first lets have a more realistic code:
function someApiCall(){
return new Promise(function(resolve, reject){
setTimeout(()=>{
resolve('Hello');
})
})
}
let data = someApiCall();
console.log(data);
This is a so-called asynchronous code, when JavaScript engine executes it, someApiCall immediately returns a result, in this case pending promise:
> Promise {<pending>}
If you pay attention to the executor, you will see we needed to pass resolve and reject arguments aka callbacks. Yes, they are callbacks required by the language construct. When either of them called, promise will change its state and hence be settled. We don't call it resolved because resolving implies successful execution but a function also can error out.
How do we get the data? Well we need more callbacks, which will be called by the executor function once the promise is settled:
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
p.then((result) => {
console.log(result); // foo
}).catch((err) => {
console.log(err);
});
Why we need to pass separate callbacks? Because one will be fed to the resolve, and the other to the reject. Then callback will be called by the resolve function, the catch callback by the reject function.
Javascript engine will execute these callbacks later on its leisure, for a regular function it means when the event loop is cleared, for timeout when the time is up.
Now to answer your question, how do we get data out from a promise. Well we can't.
If you look closely, you will see we don't really get the data out but keep feeding callbacks. There is no getting data out, but passing callbacks in.
p.then((result) => {
console.log(result);
}).catch((err) => {
console.log(err);
});
Some say use await:
async function() {
let result = await p;
}
But there is a catch. We have to or wrap it in async function. Always. Why? Because Async/await is another level of abstraction or syntactic sugar, whichever you prefer, on top of promise api. That is why we can not use await directly but always wrap it in async statement.
To sum up, when we use promise or async/await we need to follow certain convention and write terse code with closely knitted callbacks. Either javascript engine or transpilers like babeljs or typescript converts these code to regular javascript to be run.
I can understand your confusion because people keep saying getting data out when talking about promises, but we don't get any data out but pass callback to be executed when the data is ready.
Hope everything is clear now.

No, you cannot without using promises or async/await etc because calling a REST API is an asynchronous operation and is non blocking.
When you make a call to a REST API, the code shouldn't wait until the API returns a value because it may take a lot of time, making program non-responsive, thus by design making a network request is categorized as an asynchronous operation.

To avoid async/await, you'll need to use another .then:
if (something) {
return fetchDataFromAPI()
.then((data) => data /* you can console.log(data) here */)
}