I'm building a generic function that will allow to update nested object properties (ideally based on inferred type - but this is nice to have rather than requirement).

On the top of that please keep in mind - I am not a Typescript expert so I would be glad for pointing out any mistakes.

So here is the example state + types for it:

type DataType = {
  knownKeyName1: {
    knownNestedKeyName1: string
    knownNestedKeyName2: string
    knownNestedKeyName3: string
    knownNestedKeyName4: string
  }
  knownKeyName2: {
    knownNestedKeyName1: string
    knownNestedKeyName2: string
    knownNestedKeyName3: string
  }
}

type SomeState = {
  data: DataType
}

const someState: SomeState = {
  data: {
    knownKeyName1: {
      knownNestedKeyName1: 'value',
      knownNestedKeyName2: 'value',
      knownNestedKeyName3: 'value',
      knownNestedKeyName4: 'value'
    },
    knownKeyName2: {
      knownNestedKeyName1: 'value',
      knownNestedKeyName2: 'value',
      knownNestedKeyName3: 'value'
    }
  }
}

And below is the function that I've already tried to build with Typescript's generics:

const setStateValue = <
  State extends { data: unknown }, // I want this to infer type from `someState` variable whenever I pass it to this function
  Prop extends [keyof State['data']],
  VarName extends keyof State['data'][Prop] // Type 'Prop' cannot be used to index type 'State["data"]'.ts(2536)
>(
  state: State,
  prop: Prop,
  varName: VarName,
  newValue: string
): void => {
  state.data[prop][varName] = newValue

  // Example usage should results with:
  // state.data[knownKeyName1][knownNestedKeyName3] = 'someNewValue'
}

JavaScript equivalent:

 const setStateValue = (state, prop, varName, newValue) => {
    state.data[prop][varName] = newValue // Object property reassignment is done on purpose because we want to update the state in reducer with `immer`
 }

Any ideas? Additionally, if that's not possible then it would be nice to at least have something like this:

const setStateValue = <
  State extends { data: DataType },
  Prop extends keyof DataType,
  VarName extends keyof DataType[Prop]
>(
  state: State,
  prop: Prop,
  varName: VarName,
  newValue: string
): void => {
  state.data[prop][varName] = newValue // but then this error pops out: 
  // Type 'string' is not assignable to type 'DataType[Prop][VarName]'.ts(2322)
}

Thanks in advance!

P.S Adding #redux-toolkit tag to give some usage context, as I want to use this setStateValue function inside generic reducer (so the state can be mutated with immer).