Calculate all permutations of union type in TypeScript

Today we discuss Permutation

This is more synthetic example which can be hardly used in production. I didn't know about it beforehand.

Anyway let's try to solve it 🚀

Iterate over union type

Using unknown, any or never we can distribute union type:

But if we try this one, we will get an error Type instantiation is excessively deep and possibly infinite.

Somehow we need to store the union element which we currently put into the tuple, and then iterate over the rest elements. For this reason let's add another Generic type variable K:

Now we face never 🧐 for all the examples https://tsplay.dev/wj5EbW

Check if type is never or not

We have never because at the last step we have nothing to iterate over in a union type. Let's have a short example:

So for never we should return empty array to fix this problem.

To prevent future errors, we shouldn't use T extends never as this distributes union type the same way as for unknown and any. So it's not what we look for.

However, if we wrap T and never in a tuple and use [T] extends [never], it will actually check for never. Let's include it in the solution:

Looks overloaded but not that hard, right? 😊

All together

Let's recap ⬇️

1. We apply Distributive Conditional Types for K which allows us to include elements into a tuple step by step
2. With K we exclude just added element from next steps using Exclude<T, K>
3. Last iteration where there is nothing in union type we return empty tuple. We do this with condition [T] extends [never]

Please check the solution with test cases: https://tsplay.dev/weexew

Have a wonderful week ahead 🚀