export function flatten<T>(arr: any[]): T[] {
    return arr.reduce(function (flat, toFlatten) {
        return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
    }, []);
}

export function indexBy<T>(arr: T[], key: keyof T | ((item: T) => string)): Record<string, T> {
    const result: Record<string, T> = {};
    for (const item of arr) {
        const k = typeof key === "function" ? key(item) : item[key];
        if (typeof k === "string") {
            result[k] = item;
        }
    }
    return result;
}

export function groupBy<T>(arr: T[], key: keyof T | ((item: T) => string)): Record<string, T[]> {
    const result: Record<string, T[]> = {};
    for (const item of arr) {
        const k = typeof key === "function" ? key(item) : item[key];
        if (typeof k === "string") {
            if (!result[k]) result[k] = [];
            result[k].push(item);
        }
    }
    return result;
}