TS to change a type based on a "config type"

changeType.ts

// original code by MadaraUchiha

// Just defines named types we can reference by key. Never used as a real value type.
interface Operations<T> {
    'delete': never;
    'nullable': T | null;
    'box': {value: T};
};

type MemberConfig<T> = keyof Operations<T>;

type ObjectConfig<T> = {
    [K in keyof T]?: MemberConfig<T[K]>
};

type OmitNevers<T> = Pick<T, {[K in keyof T]: T[K] extends never ? never : K}[keyof T]>;

type ApplyConfig<T, C extends ObjectConfig<T>> = OmitNevers<{
    [K in keyof T]: C[K] extends MemberConfig<T[K]> ? Operations<T[K]>[C[K]] : T[K]
}>;

interface OriginalType {
    foo: string;
    bar: boolean;
    baz: number;
    bux: Date;
}

// The money shot
type NewType = ApplyConfig<OriginalType, {
    foo: 'delete';
    bar: 'box';
    baz: 'nullable';
    // don't specify bux, it should pass through unchanged.
}>;

declare const x: NewType;

const a = x.foo; // error, we deleted foo
const b = x.bar.value;
const c = x.baz.toFixed(2); // error, might be calling .toFixed() on null
const d = x.bux;