test.ts

type Result = {} | undefined;

type Fn<TArg> = (value: TArg) => Result;
type MappedFn<TKeys extends string, TArg extends {}> = {
    [K in TKeys]: Fn<TArg>;
};

type MappedInline<TKeys extends string, TArg extends {}> = {
    [K in TKeys]: (value: TArg) => Result;
};

const mFn: MappedFn<'x' | 'y', number> = {
    x: (n) => n,
    y: (n) => undefined,
};

const mInline: MappedInline<'x' | 'y', number> = {
    x: (n) => n,
    y: (n) => undefined,
};

const fields: Array<'x' | 'y'> = ['x', 'y'];

const valueFn = fields.map((field) => {
    const map = mFn[field]; // Fn<number>
    const result = map(1); // Result
    return result;
});
const valueInline = fields.map((field) => {
    const map = mInline[field]; // ((value: number) => Result) | ((value: number) => Result)
    const result = map(1); // {}
    return result;
});


// ----------


type MaybeObject = {} | undefined;
const value: MaybeObject = undefined;
console.log(value.toString());