Exclude specific types of properties in TypeScript by omitting them

Currently, I am tackling a straightforward school project and have a query regarding the possibility of excluding all properties of a specific type in TypeScript.

type Student = {
    firstName: string
    lastName: string
    age: number
    gender: Gender
    courses: List<Course>
}

For instance, I aim to create a type that retains everything from Student except for "courses".

Can we construct a Type that mirrors Student but excludes all properties with a List type? It should be versatile enough to function across various types like:

type Program = {
    name: string
    errors: List<Error>
    successes: List<Success>
    warnings: List<Warning>
}

This scenario would result in a type containing only { name: string }

If anyone has insights on achieving this, or if it's even feasible, perhaps through conditional types, Omit if List?

All assistance is highly appreciated!

Update: including code example:

type WithoutList<T> = {
    [K in keyof T]: T[K] extends List<any> ? never : T[K]
}

const select = <a, b extends keyof WithoutList<a>>(arg: a, ...keys: b[]) => {
    return null!
}

const s1: Student = {
    firstName: 's',
    lastName: 's',
    age: 22,
    gender: 'female',
    courses: List<Course>(),
}

select(s1, 'courses') // courses should not be available here!

Answer №1

To suppress specific keys, you can utilize key remapping in mapped types in TypeScript:

type WithoutList<T> = {
  [K in keyof T as T[K] extends List<any> ? never : K]: T[K]
};

This approach works effectively for the types Student and Program, as demonstrated below:

type StudentWithoutList = WithoutList<Student>;
/* type StudentWithoutList = {
    firstName: string;
    lastName: string;
    age: number;
    gender: Gender;
} */

type ProgramWithoutList = WithoutList<Program>;
/* type ProgramWithoutList = {
    name: string;
} */

The same logic applies to the behavior of your select() function:

declare const s1: Student;
select(s1, "age"); // allowed
select(s1, 'courses') // not permitted

You can test this code in a TypeScript playground here.

Answer №2

A combination of a mapped type and a conditional type that results in `never` when the type is an extension of `List` will give you the desired outcome.

Assuming `Gender`, `List`, and `Course` are not defined in your code, I will use `Array` and basic data types for demonstration:

type Student = {
    firstName: string
    lastName: string
    age: number
    courses: Array<string>
}

type WithoutList<T> = {
    [K in keyof T]: T[K] extends Array<any> ? never : T[K]
}

type StudentWithoutCourse = WithoutList<Student>;

See Demo

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