Utility types for constructing a type from an existing type, including only properties that are of a specified type. Can be applied recursively to child objects.

filter_props_by_type.ts

type FilteredKeys<T, U> = { [P in keyof T]: T[P] extends U ? P : never }[keyof T];

enum Color {
    Red, Green, Blue,
}

type PropsOfType<T, U> = {
    [K in FilteredKeys<T, U>]: T[K]
};

type TestType = {
    a: boolean;
    b: number;
    c: string;
    d: Color;
    e: {
        one: boolean;
        two: number;
    }
}

type FilteredKeysRecursive<T, U> = { [P in keyof T]: T[P] extends U ? P : T[P] extends Record<string, unknown> ? P : never }[keyof T];

type PropsOfTypeRecursive<T, U> = {
    [K in FilteredKeysRecursive<T, U>]: T[K] extends Record<string, unknown>
    ? PropsOfTypeRecursive<T[K], U>
    : T[K]
};

type TestTypeBoolProps = PropsOfType<TestType, boolean>;
type TestTypeBoolPropsRecursive = PropsOfTypeRecursive<TestType, boolean>;

const testObj1: TestTypeBoolProps = {
    a: false
}
const testObj2: TestTypeBoolPropsRecursive = {
    a: false,
    e: {
        one: false,
        two: 5, // expected error
    }
}