geninq/src/functions/single.ts

A JavaScript version of the Linq library for Generators

import { IsString } from "@paulpopat/safe-type";
import { Build, Promised } from "./utils";

declare global {
  interface Generator<T = unknown, TReturn = any, TNext = unknown> {
    single(): T;
    single(allow_empty: "or-default"): T | undefined;
    single(predicate: (item: T) => boolean): T;
    single(
      predicate: (item: T) => boolean,
      allow_empty: "or-default"
    ): T | undefined;
    single<TRes extends T>(predicate: (item: T) => item is TRes): TRes;
    single<TRes extends T>(
      predicate: (item: T) => item is TRes,
      allow_empty: "or-default"
    ): TRes | undefined;
  }

  interface AsyncGenerator<T = unknown, TReturn = any, TNext = unknown> {
    single(): Promise<T>;
    single(allow_empty: true): Promise<T | undefined>;
    single(predicate: (item: T) => Promised<boolean>): Promise<T>;
    single(
      predicate: (item: T) => Promised<boolean>,
      allow_empty: true
    ): Promise<T | undefined>;
    single<TRes extends T>(predicate: (item: T) => item is TRes): Promise<TRes>;
    single<TRes extends T>(
      predicate: (item: T) => item is TRes,
      allow_empty: "or-default"
    ): Promise<TRes | undefined>;
  }
}

Build(
  "single",
  function (predicate, allow_empty) {
    if (IsString(predicate)) {
      allow_empty = predicate as "or-default";
      predicate = (i): i is unknown => true;
    } else if (!predicate) {
      predicate = (i): i is unknown => true;
    }

    const matches = [];
    for (const item of this) {
      if (predicate(item)) {
        matches.push(item);
        if (matches.length > 1) {
          throw new Error("Too many matches in sequence");
        }
      }
    }

    if (matches.length === 1) {
      return matches[0];
    }

    if (allow_empty) {
      return undefined;
    }

    throw new Error("No matching elements in sequence.");
  },
  async function (predicate, allow_empty) {
    if (IsString(predicate)) {
      allow_empty = predicate as "or-default";
      predicate = (i): i is unknown => true;
    } else if (!predicate) {
      predicate = (i): i is unknown => true;
    }

    const matches = [];
    for await (const item of this) {
      if (await Promise.resolve(predicate(item))) {
        matches.push(item);
        if (matches.length > 1) {
          throw new Error("Too many matches in sequence");
        }
      }
    }

    if (matches.length === 1) {
      return matches[0];
    }

    if (allow_empty) {
      return undefined;
    }

    throw new Error("No matching elements in sequence.");
  }
);