RocketChat/Rocket.Chat

import { isEmptyArray } from './comparisons';

const isNullDocument = (doc: unknown): doc is undefined | null => doc === undefined || doc === null;

const isRecordDocument = (doc: unknown): doc is Record<string, unknown> =>
    doc !== undefined && doc !== null && (typeof doc === 'object' || typeof doc === 'function');

const isIndexedByNumber = <T>(value: unknown, isIndexedByNumber: boolean): value is T[] => Array.isArray(value) || isIndexedByNumber;

export const createLookupFunction = <T>(key: string): ((doc: T) => unknown[]) => {
    const [first, rest] = key.split(/\.(.+)/);

    if (!rest) {
        return <T>(doc: T): unknown[] => {
            if (isNullDocument(doc) || !isRecordDocument(doc)) {
                return [undefined];
            }

            return [doc[first]];
        };
    }

    const lookupRest = createLookupFunction(rest);
    const nextIsNumeric = /^\d+(\.|$)/.test(rest);

    return <T>(doc: T): unknown[] => {
        if (isNullDocument(doc) || !isRecordDocument(doc)) {
            return [undefined];
        }

        const firstLevel = doc[first];

        if (isEmptyArray(firstLevel)) {
            return [undefined];
        }

        const docs = isIndexedByNumber(firstLevel, nextIsNumeric) ? firstLevel : [firstLevel as T];
        return Array.prototype.concat.apply([], docs.map(lookupRest));
    };
};