Can we specify a more specific type for an argument in Typescript based on another argument in a function?

Question

Can we specify a more specific type for an argument in Typescript based on another argument in a function?

I'm in the process of developing a compact DSL for filter operations and crafting helper methods to support it.

Suppose I have a function

const equals = (left, right) => {}

This function needs to be typed so that the left value is a field within an object, while the right is a value of the same type as that object.

If I write

const equals = <T> (left: keyof T, right: T[keyof T]) => {}

, it almost works but the type of right is limited to the types available on T, rather than just the type of left.

The desired functionality can be achieved with the following:

const equals = <T, F extends keyof T>(left: F, right: T[F])

However, this setup requires two generic parameters, disrupting the type inference flow for the surrounding code. Ideally, I would like to infer the type of the second parameter based on the first parameter. Is there a way to accomplish this?

Thank you

typescript

Answer 1

Answer №1

Here is a potential solution to your query. Consider transferring some of the type validation logic to smart constructors for better organization.

// model
type PickPropertiesOfType<T, A> = Pick<
  A,
  {
    [K in keyof A]: A[K] extends T ? K : never;
  }[keyof A]
>;

export type Filter<A> =
  | { kind: "Equals"; field: string & keyof A; value: A[string & keyof A] }
  | { kind: "EqualsField"; field: string & keyof A; value: string & keyof A };

export const equalsOtherField = <A, K extends string & keyof PickPropertiesOfType<A[K], A>>(
  field: string & K,
  value: string & keyof PickPropertiesOfType<A[K], A>
): Filter<A> => ({
  kind: "EqualsField",
  field,
  value: value,
});

export const equals = <A, K extends string & keyof A>(field: string & K, value: A[K]): Filter<A> => ({
  kind: "Equals",
  field,
  value: value,
});

// example
type ExampleEntity = {
    author: string;
    publisher: string;
    year: number;
}

const exampleEntity: ExampleEntity = {
    author: "Foo",
    publisher: "Foobar",
    year: 2000
}

const filterArr: Filter<ExampleEntity>[] = [
  equals("author", "Bar"),
  equalsOtherField("author", "publisher"),
];

Answer 2

Here is a potential solution to your query. Consider transferring some of the type validation logic to smart constructors for better organization.

// model
type PickPropertiesOfType<T, A> = Pick<
  A,
  {
    [K in keyof A]: A[K] extends T ? K : never;
  }[keyof A]
>;

export type Filter<A> =
  | { kind: "Equals"; field: string & keyof A; value: A[string & keyof A] }
  | { kind: "EqualsField"; field: string & keyof A; value: string & keyof A };

export const equalsOtherField = <A, K extends string & keyof PickPropertiesOfType<A[K], A>>(
  field: string & K,
  value: string & keyof PickPropertiesOfType<A[K], A>
): Filter<A> => ({
  kind: "EqualsField",
  field,
  value: value,
});

export const equals = <A, K extends string & keyof A>(field: string & K, value: A[K]): Filter<A> => ({
  kind: "Equals",
  field,
  value: value,
});

// example
type ExampleEntity = {
    author: string;
    publisher: string;
    year: number;
}

const exampleEntity: ExampleEntity = {
    author: "Foo",
    publisher: "Foobar",
    year: 2000
}

const filterArr: Filter<ExampleEntity>[] = [
  equals("author", "Bar"),
  equalsOtherField("author", "publisher"),
];

Can we specify a more specific type for an argument in Typescript based on another argument in a function?

Answer №1

Similar questions

Master Angular Autocompletion

Can dynamic string types be declared in Typescript?

Difficulties in Networking Requests Following Event Emitter Notification in an Angular Application

What is the reason for the inability of `Array<Value>, Value` to properly retrieve the type of the array value?

Allowing the use of a string as a parameter in a Typescript constructor

Unable to impose a restriction on the number input field limit

Uh-oh! A circular dependency has been detected in the Dependency Injection for UserService. Let's untangle this web and fix the issue!

Uncovering the perfect body proportions using Webpack and SystemJS

The Console.log function first displays an Object, and then changes to display an Array containing a single object when clicked

The Vue $refs Object is classified as 'unidentified' in nature

Problems with the zoom functionality for images on canvas within Angular

Steps for utilizing a function from the parent component to initialize TinyMCE

Display Module within Component using Angular 5

Angular Delight: Jaw-Dropping Animation

Is there a way to specify this component without it being nested within the parent element?

Prisma DB is a versatile database that excels in handling m-n

Retrieving data from notifications without having to interact with them (using Firebase Cloud Messaging and React Native)

Tips for validating and narrowing a type in TypeScript using isNull instead of isNonNullable

Using TypeScript to define callback functions within the Cordova.exec method

Expanding a class in Typescript by adding a function with the same name but varying properties and types