poor-validation.ts

function hasKey<T extends object, K extends string>(x: T, key: K): x is T & { [_ in K]: unknown } {
    return key in x;
}

type TypeOfs = {
    string: string;
    number: number;
    object: object;
};

function hasShapeFlat<T extends object, S extends Record<string, keyof TypeOfs>>(
    x: T,
    shape: S
): x is T & { [P in keyof S]: TypeOfs[S[P]] } {
    return Object.entries(x).every(([key, val]) => typeof val === shape[key]);
}

const x: object = {};

if (hasKey(x, 'a')) {
    const { a } = x;
}

if (hasShapeFlat(x, { a: 'string', b: 'number' })) {
    const b = x.b * 2;
    const a = x.a.startsWith('a');
}