How to get response type depending on the input params?

Simple example of a normal JS function with generics:

const example = <T extends unknown>(params: T): T => {
  return params
}

// TYPE: const params: readonly ["Hello", "World"]
const params = ['Hello', 'World']

// TYPE: const result: readonly ["Hello", "World"]
const result = example(params)

// TYPE: const str1: string, const str2: string
const [str1, str2] = result

const example = <T extends unknown>(params: T): T => {
  return params
}

// TYPE: const params: readonly ["Hello", "World"]
const params = ['Hello', 'World']

// TYPE: const result: readonly ["Hello", "World"]
const result = example(params)

// TYPE: const str1: string, const str2: string
const [str1, str2] = result

This is what I want to achieve with TRPC. If I send some params I want my respone type be able to change depending on the params. How can I achieve that?

// TYPE: const params: readonly ["Hello", "World"]
const params = ['Hello', 'World']

const { data } = trpc.router.model.example(params)

// I want the following type returned based on the params 
// TYPE: const result: readonly ["Hello", "World"]
const result = data;

// I want the same result as in the simple example
// TYPE: const str1: string, const str2: string
const [str1, str2] = result

// TYPE: const params: readonly ["Hello", "World"]
const params = ['Hello', 'World']

const { data } = trpc.router.model.example(params)

// I want the following type returned based on the params 
// TYPE: const result: readonly ["Hello", "World"]
const result = data;

// I want the same result as in the simple example
// TYPE: const str1: string, const str2: string
const [str1, str2] = result

5 Replies

A. Perkamentus•2y ago

I already checked this page in the docs, but this does not describe my use case (https://trpc.io/docs/infer-types)

Nick•2y ago

Simplest way would be using a discriminated union: https://www.typescriptlang.org/docs/handbook/unions-and-intersections.html#discriminating-unions

Handbook - Unions and Intersection Types

How to use unions and intersection types in TypeScript

Nick•2y ago

But that's not exactly what you're showing in the TS example

Nick•2y ago

Maybe a router factory if it's for a known set of entities and you don't mind splitting them up among routes: https://dev.to/nicklucas/trpc-patterns-router-factories-and-polymorphism-30b0

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Nick•2y ago

True covariance of inputs in TS is a challenge because it can be achieved via method overloads, but that's not type-safe on the inside