How to modify a function type with declare module without creating an overload in Typescript?

Question

I have a library "foo" that implements a function "bar" like this

declare const bar = (baz: string) => void

I want to override that type to be something different, like

declare const bar = (baz: number) = void

So I do this

declare module "foo" {
  declare const bar = (baz: number) = void
}

But when I do this, I just end with a function overload and it allows both number and string to be passed.

How can I make that function be of the type I want to "override" without creating a function overload?

Answer 1 · 2025-11-29 21:18:56Z

This is syntactically invalid pseudocode. And you can't redeclare consts at all; it doesn't result in a merge, just an error about redeclaration of a const. Please fix this to be a minimal reproducible example and maybe someone can help.

That being said, you cannot replace declarations, only merge with them. And merging a function or a method (but not a const) results in an overload. So assuming I do understand what you're asking for, it's impossible. You would need to locally fork/modify/replace the declaration in question. Maybe if ms/TS#19064 were ever implemented you could remove the existing type and add a new one. But it's not implemented, so it's not possible at all.

Collectives™ on Stack Overflow

How to modify a function type with declare module without creating an overload in Typescript?

1 Reply 1

Your Reply

Collectives™ on Stack Overflow

How to modify a function type with declare module without creating an overload in Typescript?

1 Reply 1

Your Reply

Sign up or log in

Post as a guest