Exploring the versatility of functions through annotations

Question

Exploring the versatility of functions through annotations

Hello community! Seeking assistance for annotating a polymorphic function. I have multiple types that share a common key "type" along with other keys:

type T1 = { type: 't1', someKey: number };
type T2 = { type: 't2', anotheKey: string };
type T3 = { type: 't3', andAnotherKey: boolean };
type T = T1 | T2 | T3;

I also have a map of functions. Each key in this map corresponds to the "type" field, and the value is a function taking an argument of the respective type:

const TYPE_FN_MAP = {
  t1: (argT1: T1) => argT1.someKey,
  t2: (argT2: T2) => argT2.anotheKey,
  t3: (argT3: T3) => argT3.andAnotherKey,
} as const

How do I annotate the polymorphic function:

const polymorphicFn = (argT) => TYPE_FN_MAP[argT.type](argT);

The idea is that when adding a new type, simply extend the TYPE_FN_MAP with a function taking an argument of that type, while keeping the polymorphicFn function unchanged. If the "type" field is forgotten in a new type, TypeScript will flag an error in the polymorphicFn function.

Playground

typescript

Answer 1

Answer №1

Are you in search of something similar to this?

type ARG_TYPE = typeof TYPE_FN_MAP;

function customTest<K extends keyof ARG_TYPE>(event: K, ...arg: Parameters<ARG_TYPE[K]>) : ReturnType<ARG_TYPE[K]> {
  // Implement logic here
  return {} as unknown as ReturnType<ARG_TYPE[K]>;
}

customTest('test1', { type: 'test1', someValue: 10 })

Link

If a valid parameter is not provided for the first parameter, an error may occur like this:

customTest('test1', { someValue: 10 })
// Error: Argument of type '{ someValue: number; }' is not assignable to parameter of type 'Test1'.
  Property 'type' is missing in '{ someValue: number; }' but required in 'Test1'.(2345)

However, duplication may occur if the key 'type' is provided both within the specified type and in TYPE_FN_MAP.

A simpler approach could be:

type TYPE_FN_MAP = {
  test1: () => { someValue: number },
  test2: (param: boolean) => { anotherValue: string },
  test3: () => { andAnotherValue: boolean },
} 

function simplifiedTest<K extends keyof TYPE_FN_MAP>(event: K, ...arg: Parameters<TYPE_FN_MAP[K]>) : ReturnType<TYPE_FN_MAP[K]> {
  // Perform operation here
  return {} as unknown as ReturnType<TYPE_FN_MAP[K]>;
}

const result = simplifiedTest('test1');
console.log(result.someValue)

simplifiedTest('test2') // Expected 2 arguments, but received only 1.(2554)
simplifiedTest('test2', true)

Link

Answer 2

Are you in search of something similar to this?

type ARG_TYPE = typeof TYPE_FN_MAP;

function customTest<K extends keyof ARG_TYPE>(event: K, ...arg: Parameters<ARG_TYPE[K]>) : ReturnType<ARG_TYPE[K]> {
  // Implement logic here
  return {} as unknown as ReturnType<ARG_TYPE[K]>;
}

customTest('test1', { type: 'test1', someValue: 10 })

Link

If a valid parameter is not provided for the first parameter, an error may occur like this:

customTest('test1', { someValue: 10 })
// Error: Argument of type '{ someValue: number; }' is not assignable to parameter of type 'Test1'.
  Property 'type' is missing in '{ someValue: number; }' but required in 'Test1'.(2345)

However, duplication may occur if the key 'type' is provided both within the specified type and in TYPE_FN_MAP.

A simpler approach could be:

type TYPE_FN_MAP = {
  test1: () => { someValue: number },
  test2: (param: boolean) => { anotherValue: string },
  test3: () => { andAnotherValue: boolean },
} 

function simplifiedTest<K extends keyof TYPE_FN_MAP>(event: K, ...arg: Parameters<TYPE_FN_MAP[K]>) : ReturnType<TYPE_FN_MAP[K]> {
  // Perform operation here
  return {} as unknown as ReturnType<TYPE_FN_MAP[K]>;
}

const result = simplifiedTest('test1');
console.log(result.someValue)

simplifiedTest('test2') // Expected 2 arguments, but received only 1.(2554)
simplifiedTest('test2', true)

Link

Exploring the versatility of functions through annotations

Answer №1

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