Repository navigation
Unable to append a type to an existing tuple #31286
Description
Activity
Your playground link links to an empty Playground.
sorry about that, link in original post fixed.
Duplicates #5453 and fixed in TS3.0 by #24897 via tuples in rest/spread positions:
type Cons<H, T extends any[]> = ((h: H, ...t: T) => void) extends (( ...r: infer R ) => void) ? R : never; function combine<T, U extends any[] | [any]>(item: T, items: U): Cons<T, U> { const ret = items.slice() as Cons<T, U>; ret.unshift(item); return ret; } const result = combine("hello", ["goodbye", 123, true]); // const result: [string, string, number, boolean]
Note: this should be called "prepend", not "append". Appending to and concatenating tuples is an open issue (#26058) with workarounds of various levels of hackiness available.
Reacted by Roaders and Ran LottemThat's an ...interesting way of fixing this!
This is a hack though right? It's a very roundabout way of fixing the issue IMHO.
Very appreciative of the solution though :-)
It's a hack in that it's roundabout, but that's probably a separate issue #26113. But this definition of
Consuses all supported and documented language features, so it's not a hack in the sense that "it just happens to work".- locked as resolved and limited conversation to collaborators
on Oct 21, 2025
TypeScript Version: 3.2.4
Search Terms:
Tuple, merge, combine
Code
Expected behavior:
result should be typed as
[string, string, number, boolean].Actual behavior:
Compile error:
A rest element type must be an array type.
Playground Link:
Link
Related Issues: