/**
 * fast O(n) non-mutating convert an array to a `Map` with keys
 * defined by a property, dot-prop path, or extractor function
 *
 * An alternate usage exists when given an array of string or number:
 * in that case the return Map maps the value to `true`. May be better
 * to use a Set for these cases.
 */
export declare function mapkeyby<ObjectType = string | number | undefined | null>(objArray: ObjectType[] | undefined): Map<ObjectType, boolean>;
export declare function mapkeyby<ObjectType extends object, Key extends keyof ObjectType>(objArray: ObjectType[] | undefined, key: Key): Map<ObjectType[Key], ObjectType>;
export declare function mapkeyby<ObjectType, KeyType = any>(objArray: ObjectType[] | undefined, key: string | number | symbol): Map<KeyType, ObjectType>;
export declare function mapkeyby<ObjectType, KeyType>(objArray: ObjectType[] | undefined, extractor: (obj: ObjectType) => KeyType): Map<KeyType, ObjectType>;