TypeScript: Turn an Array of Values into a Type Union
I found the answer I was looking for here: https://dev.to/shakyshane/2-ways-to-create-a-union-from-an-array-in-typescript-1kd6
I wasn't sure that it would be possible. In fact, it turns out that the solution is far more impressive than I expected.
What follows cuts to the core part of the article that I need solved my particular problem.
I want to have an object type with specific keys and I want a static array of
those keys, but I don't want to have the same list of keys written out
twice in case I mistakenly make them differ in the future.
I want to be sure that 'keys' contains every member of type 'K' exactly once and vice-versa. I want to use 'keys' to build an object of type 'O', so I can't just rely on using 'Object.keys' on an existing object.
Use 'as const' to make the type of 'keys' be interpreted as a readonly tuple of strings, whereas before it was an array, 'K', which could have any members of 'K' anywhere zero or more times each.
Because 'keys' is a value, it needs to be prefixed with 'typeof' to use its type in a type expression.
By indexing a tuple type with a specific number, you can get the type at that position in the tuple.
The incredible part: By indexing the tuple type with a union of numbers, you get the union of the types at those positions. Therefore by indexing 'typeof keys', which is a tuple, with 'number', which is the union of all numbers, you get the union of all strings in 'keys'.