Combiner function arguments have unknown type

[yndx-madfriend]

Hi! Sorry if this was asked before. The closest issue I found was #547 and #559, if this one is a duplicate, feel free to close it.

Since we updated from 3.x to 4.1.8, we got a lot of errors with combiner function arguments getting typed as any.

I created a small playground to reproduce the issue here:

import { createSelector } from 'reselect';

createSelector(
    (_state) => 5,
    // argument five below has type "unknown"
    (five) => ({ five })
)

createSelector(
    (_state): 5 => 5,
    // argument five below has type "unknown"
    (five) => ({ five })
)

const get5 = (_state): 5 => 5;

createSelector(
    get5,
    // argument five below has type 5
    (five) => ({ five })
)

Is this the intended behavior of reselect typings?

[markerikson]

You must provide correctly typed arguments for all of the input functions. As of Reselect 4.1, Reselect infers everything from those arguments.

Define (_state: RootState) => 5 (and similarly for any other arguments), and it will all work correctly!

[yndx-madfriend]

You must provide correctly typed arguments for all of the input functions. As of Reselect 4.1, Reselect infers everything from those arguments.

Define (_state: RootState) => 5 (and similarly for any other arguments), and it will all work correctly!

I do not understand why, with the new typings of createSelector, it makes such a difference (whether to have a function inlined or defined separately). Also, as I said, it all used to work in v3, and this new requirement of having separate definitions is not explicitly stated neither in changelog nor in readme.

[markerikson]

It's not about the "inline" or "defined separately".

It's literally that you must tell Reselect what the types of the arguments are.

// ❌ FAILS - no type for `state`
createSelector(
  (state) => state.x,
  output
)

// ✅ WORKS - TS knows the type of `state`
createSelector(
  (state: RootState) => state.x,
  output
)

Reselect's types now infer everything from those arguments, and tbh I'm not sure how this ever would have worked even with the old types if you don't provide any argument types at all.

[yndx-madfriend]

Thank you for clarifications @markerikson , it seems to work correctly now with declared types of arguments. 🎉