Today we discuss Promise.all
This is helpful in case we need to wait for all the values (e.g. we pass
[Promise<number>, 42, Promise<string>] and expect to have
Promise<[number, 42, string]> at the end).
Let’s solve it 🚀
Previously, one of the challenges in easy category was Unwrapping the Promises where we imitated
await in types:
This will help to iterate over tuple elements and replace
Promise with the resolved type.
We already know how to iterate over tuple elements with Making object out of tuple but here we have the difference:
- If we want to have elements of tuple inside the object type, we use Mapped Types:
- If we need to work with tuple elements and leave the structure as is, we use Type inference in conditional types with Rest elements in Tuples:
As we need to transform the elements inside the tuple but still need to have the tuple at the end, we will use the second approach. Let’s go over all elements and unwrap it with
The only thing we need to do is to apply it to
Let’s see what happened in Playground. We still have failed tests. But why?
If we hover over constants we will see that the result is
Promise<>, but we expected it to have elements’ types inside a tuple.
This happens because
as const converts an array to a tuple. For tuples
T extends [infer H, ...infer Tail] ? true : false returns
false because we forgot
But instead of changing
PromiseReducer let’s have a look at
T is inferred as
(number | Promise<number>) instead of
[number, number, Promise<number>]. There is an approach to have values as tuple instead of array in types:
- We added
- We say that
readonlyto work with tuples only
- We added Generic Constrain to use spread in tuples
That’s it 🔥
Solution is available on Playground
Have a wonderful evening 🌇typescriptmedium