Type constraints on arguments in Typescript functions

Question

Type constraints on arguments in Typescript functions

I've been struggling with this issue for quite some time,

function processEntity<
  T extends Record<string, unknown>,
  U extends keyof T = keyof T,
  V extends Pick<T, U> = Pick<T, U>
>(arg: {keys: Array<U>; resolver: (doc: V) => void}) {
  arg.resolver(...)
}

type Cat = {
  name: string
  age: number
}

processEntity<Cat>({
  keys: ['name'],
  resolver: (doc) => {
    // doc.name should exist
    // doc.age should be undefined
  },
})

As shown above, there is a function called processEntity that takes an object as an argument.

My goal is to have the

type of the argument in arg.resolver

depend on the type of arg.keys. For example, if I pass ['name'] to the keys: field, then doc.age should not be accessible because 'age' was missing in the keys field.

I have looked at some related questions listed below but they have not been helpful, so please provide assistance :pray:

How to define a function's argument type dependent on string argument in Typescript?
Typescript make one parameter type depend on the other parameter
Declaring dependent argument types for optional arguments with conditional types
Typescript. Correctly typing function parameter that depends on property in the same object literal

typescript

Answer 1

Answer №1

You are on the right track. The key issue here is that Typescript does not offer support for partial inference within functions. This essentially means that you have to either specify all type parameters or allow Typescript to infer all of them - there is no middle ground where you can specify some and let TS figure out the rest. Specifically, you cannot define T and expect TS to infer U.

Fortunately, there are a few workarounds to address this limitation. One approach is to employ function currying to define T in the initial call and then let TS infer U in the subsequent call:

function processEntity<
    T extends Record<string, unknown>>() {
    return function <
        U extends keyof T = keyof T
    >(arg: { keys: Array<U>; resolver: (doc: Pick<T, U>) => void }) {
        //   arg.resolver(...)
    }
}

Playground Link

Alternatively, you can introduce a dummy field to serve as an inference point for T, even though the actual value assigned to it is irrelevant:

function processEntity<
    T extends Record<string, unknown>,
    U extends keyof T = keyof T,
    >(arg: { type: T, keys: Array<U>; resolver: (doc: Pick<T, U>) => void }) {
    //arg.resolver(...)
}

Playground Link

Answer 2

You are on the right track. The key issue here is that Typescript does not offer support for partial inference within functions. This essentially means that you have to either specify all type parameters or allow Typescript to infer all of them - there is no middle ground where you can specify some and let TS figure out the rest. Specifically, you cannot define T and expect TS to infer U.

Fortunately, there are a few workarounds to address this limitation. One approach is to employ function currying to define T in the initial call and then let TS infer U in the subsequent call:

function processEntity<
    T extends Record<string, unknown>>() {
    return function <
        U extends keyof T = keyof T
    >(arg: { keys: Array<U>; resolver: (doc: Pick<T, U>) => void }) {
        //   arg.resolver(...)
    }
}

Playground Link

Alternatively, you can introduce a dummy field to serve as an inference point for T, even though the actual value assigned to it is irrelevant:

function processEntity<
    T extends Record<string, unknown>,
    U extends keyof T = keyof T,
    >(arg: { type: T, keys: Array<U>; resolver: (doc: Pick<T, U>) => void }) {
    //arg.resolver(...)
}

Playground Link

Type constraints on arguments in Typescript functions

Answer №1

Similar questions

Prevent modal from closing when clicking outside in React

Tips for associating an id with PrimeNg menu command

Is Typescript capable of converting ES6 code to ES5 during transpilation?

Creating a String Array and linking it to an Input Field

Is there a way to import a module generated by typescript using its name directly in JavaScript?

Establishing a pair of separate static directories within Nest

Altering the properties of a specified element within TestBed using the overrideComponent method

Elements constrained by themselves in a rest parameter with generic types

Configuring eslint-import-resolver-typescript in a multi-package repository

Transform JSON into a TypeScript interface with a specialized Date method

What is the source of the compiler options in tsconfig.json?

Analyzing past UTC date times results in a peculiar shift in time zones

Testing NestJS Global ModulesExplore how to efficiently use NestJS global

Updating the checkbox status in Angular when the radio button value is changed

What is the best way to implement dotenv in a TypeScript project?

Using TypeScript to implement Angular Draggable functionality within an ng-template

Issue encountered while executing jest tests - unable to read runtime.json file

Error with TypeScript Compiler in Angular 2

I am encountering difficulties while attempting to import Typescript files. Upon compiling them into Javascript, I am faced with errors in the web browser, specifically the issue of "exports is not defined"

Transforming an array of JavaScript objects into arrays of key-value pairs based on a specific property with ES6 syntax