What causes the second parameter of the function to always appear as "never"?

Question

What causes the second parameter of the function to always appear as "never"?

I am looking to create a simple generic interface for a function that accepts an object's interface, where the first argument is the key of the object and the second argument is another key within that object.

Here is my proposed 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
  }
}

However, I encountered the following issue:

https://i.sstatic.net/NR0o0.jpg

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

javascript typescript function generics types

Answer 1

Answer №1

It's hard to put into words, but I have a feeling it will do the trick

interface Experiment<E> {
  <P1 extends keyof E, P2 extends keyof E[P1]>(param1: P1, param2: P2): void;
}

const transformationFunctions = {
  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 executeExperiment: Experiment<typeof transformationFunctions> = ()=> {}
executeExperiment('ms', 'h');

Answer 2

It's hard to put into words, but I have a feeling it will do the trick

interface Experiment<E> {
  <P1 extends keyof E, P2 extends keyof E[P1]>(param1: P1, param2: P2): void;
}

const transformationFunctions = {
  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 executeExperiment: Experiment<typeof transformationFunctions> = ()=> {}
executeExperiment('ms', 'h');

What causes the second parameter of the function to always appear as "never"?

Answer №1

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