Add GetLength, Tail, LastIndex, and Last

[fabiancook]

We're able to get the length of an array using the following:

type GetLength<T extends any[]> = T extends { length: infer L } ? L : never

We can also get everything but the first element of an array using (I did name this Tail, but seems better to be WithoutFirst, or something else):

type WithoutFirst<T extends any[]> = ((...test: T) => void) extends ((sacrifice: any, ...rest: infer R)=>void) ? R : never;

Once we have a length, and a way to tail, then we're able to get the last index of a typed array:

type LastIndex<T extends any[]> = GetLength<T> extends 1 ? 0 : GetLength<WithoutFirst<T>>;

And once we have the last index, we can get the last element

type Last<T extends any[]> = T[LastIndex<T>];

Here is a little demo of it...

http://www.typescriptlang.org/play/#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

[joonhocho]

@fabiancook isn't typeof length a number? This only works for tuple types not arrays?

[fabiancook]

These would all only apply to fixed length arrays (as you mentioned, tuples)

For non fixed length arrays, it would result in number

I've shown the two modes here

http://www.typescriptlang.org/play/#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-a9WgghvMg+jwnGHCWAWUBvGslKRFAcEtH+n4AKpOGeW6-h+8yAcBp7njuSAvDOkHjGseLYtgQA

[fabiancook]

I think as long as this is clear within the documentation, it shouldn't be an issue, everything works as I would expect it to, I haven't seen a clear way to get the last/last index of a fixed length array before, so think it would be a great addition to this set of types