@lwc/shared/types/fields.d.ts

export declare type HiddenField<T> = {
    __type: T;
};
export declare function createHiddenField<T = unknown>(key: string, namespace: string): HiddenField<T>;
export declare function setHiddenField<T>(o: any, field: HiddenField<T>, value: T): void;
export declare function getHiddenField<T>(o: any, field: HiddenField<T>): T | undefined;