When to use promise.all()?

I'm not sure anyone has really given the most general purpose explanation for when to use Promise.all() (and when not to use it):

What is(are) the correct scenario(s) to use promise.all()

Promise.all() is useful anytime you have more than one promise and your code wants to know when all the operations that those promises represent have finished successfully. It does not matter what the individual async operations are. If they are async, are represented by promises and your code wants to know when they have all completed successfully, then Promise.all() is built to do exactly that.

For example, suppose you need to gather information from three separate remote API calls and when you have the results from all three API calls, you then need to run some further code using all three results. That situation would be perfect for Promise.all(). You could so something like this:

Promise.all([apiRequest(...), apiRequest(...), apiRequest(...)]).then(function(results) {
    // API results in the results array here
    // processing can continue using the results of all three API requests
}, function(err) {
    // an error occurred, process the error here
});

Promise.all() is probably most commonly used with similar types of requests (as in the above example), but there is no reason that it needs to be. If you had a different case where you needed to make a remote API request, read a local file and read a local temperature probe and then when you had data from all three async operations, you wanted to then do some processing with the data from all three, you would again use Promise.all():

Promise.all([apiRequest(...), fs.promises.readFile(...), readTemperature(...)]).then(function(results) {
    // all results in the results array here
    // processing can continue using the results of all three async operations
}, function(err) {
    // an error occurred, process the error here
});

On the flip side, if you don't need to coordinate among them and can just handle each async operation individually, then you don't need Promise.all(). You can just fire each of your separate async operations with their own .then() handlers and no coordination between them is needed.

In addition Promise.all() has what is called a "fast fail" implementation. It returns a master promise that will reject as soon as the first promise you passed it rejects or it will resolve when all the promises have resolved. So, to use Promise.all() that type of implementation needs to work for your situation. There are other situations where you want to run multiple async operations and you need all the results, even if some of them failed. Promise.all() will not do that for you directly. Instead, you would likely use something like Promise.settle() for that situation. You can see an implementation of .settle() here which gives you access to all the results, even if some failed. This is particularly useful when you expect that some operations might fail and you have a useful task to pursue with the results from whatever operations succeeded or you want to examine the failure reasons for all the operations that failed to make decisions based on that.

Are there any best practices to use promise.all()? Should it be ideally used only if all of the promise objects are of the same or similar types?

As explained above, it does not matter what the individual async operations are or if they are the same type. It only matters whether your code needs to coordinate them and know when they all succeed.

---

It's also useful to list some situations when you would not use Promise.all():

1. When you only have one async operation. With only one operation, you can just use a .then() handler on the one promise and there is no reason for Promise.all().
2. When you don't need to coordinate among multiple async operations.
3. When a fast fail implementation is not appropriate. If you need all results, even if some fail, then Promise.all() will not do that by itself. You will probably want something like Promise.allSettled() instead.
4. If your async operations do not all return promises, Promise.all() cannot track an async operation that is not managed through a promise.

Promise.all is for waiting for several Promises to resolve in parallel (at the same time). It returns a Promise that resolves when all of the input Promises have resolved:

// p1, p2, p3 are Promises
Promise.all([p1, p2, p3])
  .then(([p1Result, p2Result, p3Result]) => {
    // This function is called when p1, p2 and p3 have all resolved.
    // The arguments are the resolved values.
  })

If any of the input Promises is rejected, the Promise returned by Promise.all is also rejected.

A common scenario is waiting for several API requests to finish so you can combine their results:

 const contentPromise = requestUser();
 const commentsPromise = requestComments();

 const combinedContent = Promise.all([contentPromise, commentsPromise])
   .then(([content, comments]) => {
     // content and comments have both finished loading.
   })

You can use Promise.all with Promise instance.