Typescript parameter type depending on another parameter

Question

I want to create generic function for setStore, the function is simple:

const setStore = <T>(store:T) => <K extends keyof T,U extends boolean>(property: K,data: U extends true? Partial<T[K]>:T[K],partialUpdate:U) => {
if (partialUpdate) {
      store[property] = { ...store[property], ...data }
    } else {
      store[property] = data // Error Type 'U extends true ? Partial<T[K]> : T[K]' is not assignable to type 'T[K]'.
    }
}

I want data's type to be T[K] or Partial<T[K]> depending on the partialUpdate parameter is true or false. But I get the error Type 'U extends true ? Partial<T[K]> : T[K]' is not assignable to type 'T[K]'. because somehow typescript can't decide whether the data type is T[K] or Partial<T[K]>

BorisTB · Accepted Answer · 2022-04-07 08:15:34Z

1

Just tell typescript that data really is a T[K].

const setStore = <T>(store:T) => <K extends keyof T,U extends boolean>(property: K,data: U extends true? Partial<T[K]>:T[K],partialUpdate:U) => {
  if (partialUpdate) {
        store[property] = { ...store[property], ...data }
  } else {
        store[property] = data as T[K]
  }
}

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answered Apr 7, 2022 at 8:15

BorisTB

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1 Comment

Xin Jin Over a year ago

Ah, ok, thanks :) it sounds like a solution. But I just try to avoid to use type assertion, because maybe it's not properly typed. And i don't know why Typescript can't figure out the proper type in this case

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