Why the second argument of function become never

user_15074237 0
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want to create simple generic interface for function, that accepts interface of object and first argument is key of object and second is key2 of object by object[key].

Here my solution

type Test<T extends Object, K extends keyof T = keyof T, V extends keyof T[K] = keyof T[K]> = (key: K, key2: V) => void

Object

const convertFunctionsMap = {
  ms: {
    h: millisecondsToHours,
    s: millisecondsToSeconds,
    m: millisecondsToMinutes
  },
  s: {
    h: secondsToHours,
    ms: secondsToMilliseconds,
    m: secondsToMinutes
  },
  m: {
    ms: minutesToMilliseconds,
    s: minutesToSeconds,
    h: minutesToHours
  },
  h: {
    ms: hoursToMilliseconds,
    s: hoursToSeconds,
    m: hoursToMinutes
  }
}

But I got

Result

CodeSandbox: https://codesandbox.io/s/empty-leaf-cuz51r?file=/src/index.ts

typescript function javascript generics types
1 Answers

i don't how to explain, but i know it works

interface Test<T> {
  <K1 extends keyof T, K2 extends keyof T[K1]>(key: K1, key2: K2): void;
}

const convertFunctionsMap = {
  ms: {
    h: 1,
    s: 2,
    m: 3
  },
  s: {
    h: 1,
    ms: 2,
    m: 3
  },
  m: {
    ms: 1,
    s: 2,
    h: 3
  },
  h: {
    ms: 1,
    s: 2,
    m: 3
  }
}
const getIn2: Test<typeof convertFunctionsMap> = ()=> {}
getIn2('ms', 'h');
user_101746510
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