expose.ts

const CUSTOM_PROPS = 'custom:properties';

function Expose(target: object, key: string) {
    let properties: string[] | undefined = Reflect.getMetadata(CUSTOM_PROPS, target);
    if (! properties) {
        properties = [];
    }

    properties.push(key);

    Reflect.defineMetadata(CUSTOM_PROPS, properties, target);
}

class Page {
    @Expose content: object;
}