First of all we need to distinguish two definitions: arrays and tuples in typescript.
Consider array as a list of unknown length, something like vector in Rust.
Consider tuple as a list of known, during the compilation, length.
In order to handle allowed keys for [true, false] tuple and there are only two of them 0 | 1 we need to write a small utility type:
type IsTuple<T> =
(T extends Array<any> ?
(T['length'] extends number
? (number extends T['length']
? false
: true)
: true)
: false)
{
type _ = IsTuple<[1, 2]> // true
type __ = IsTuple<number[]> // false
type ___ = IsTuple<{ length: 2 }> // false
}
type AllowedTupleLength<
T extends ReadonlyArray<unknown>,
Length extends number = never
> = T extends readonly [infer _, ...infer Tail]
? AllowedTupleLength<Tail, Length | Tail['length']>
: T extends readonly []
? Length
: never;
type Result = AllowedTupleLength<[0,0]> // 1 | 0
Now we need an utility which will conditionaly return allowed keys for tuple and array:
type ComputeKeys<Tuple extends any[]> =
IsTuple<Tuple[0]> extends true ? AllowedTupleLength<Tuple[0]> : keyof Tuple[0]
ComputeKeys returns 1|0 for [true, false] and all array keys for regular array.
Btw, do you want to allow using forEach, reduce, map keys as a second element in the tuple ?
Also, we need to handle all non array objects. Let's create Json type with all serializable types in a union:
type Json = | string | number | boolean | { [prop: string]: Json } | Array<Json>
type IsTuple<T> =
(T extends Array<any> ?
(T['length'] extends number
? (number extends T['length']
? false
: true)
: true)
: false)
type AllowedTupleLength<
T extends ReadonlyArray<unknown>,
Length extends number = never
> = T extends readonly [infer _, ...infer Tail]
? AllowedTupleLength<Tail, Length | Tail['length']>
: T extends readonly []
? Length
: never;
type ComputeKeys<Tuple extends any[]> =
IsTuple<Tuple[0]> extends true ? AllowedTupleLength<Tuple[0]> : keyof Tuple[0]
function handleTuple<
Elem extends Exclude<Json, any[]>, // overload signature for non array serializable values
Tuple extends Elem[]
>(tuple: [...Tuple, keyof Tuple[0]]): [...Tuple]
function handleTuple<
Elem,
NestedTuple extends Elem[],
Tuple extends [...NestedTuple][]
>(tuple: [...Tuple, ComputeKeys<Tuple>]): [...Tuple]
function handleTuple(tuple: unknown[]) {
return tuple
}
handleTuple([[true, false], 1]) // ok
handleTuple([[true, false], 2]) // expected error, index is too big
handleTuple([new Array(), 'length']) // ok
handleTuple([new Array(), 2]) // allowed because we don't know exact length of array
handleTuple([{ name: 'John', age: 32 }, 'name']) // allowed because we don't know exact length of array
Playground
Btw, since you don't know all allowed objects in a tuple, it is impossible to make it without extra function.
