@ugo-code/streamline.js/dist/esm/singleExecution.js

A utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments || [])).next());
    });
};
import { hashObject } from "./crypto";
/**
 * A map to store promises of currently active (in-flight) tasks.
 * The key is a unique hash, and the value is the promise returned by the task.
 */
const activeRequests = new Map();
/**
 * Ensures that an asynchronous task is only executed once at a time for a given key.
 *
 * If this function is called while a task with the same key is already running,
 * it will return the promise of the existing task instead of starting a new one.
 * This is useful for preventing duplicate network requests or other expensive
 * operations. Once a task is complete (either resolves or rejects), its promise is
 * removed, and the next call with the same key will trigger a new execution.
 *
 * @template TResult The expected result type of the asynchronous task.
 * @param {() => Promise<TResult>} taskFn The asynchronous function to execute.
 * @param {SerializableKey} [key] An optional unique identifier for the task.
 *   If it's an object or array, it will be JSON-stringified and hashed.
 *   If not provided, the task function's source code (`taskFn.toString()`) is
 *   hashed to generate a key. Note that this may not be unique for different
 *   function instances with identical source code.
 * @returns {Promise<TResult>} A promise that resolves or rejects with the result of the task.
 * @example
 * ```ts
 * // Example 1: Basic deduplication with a string key
 * async function fetchUser(userId: string) {
 *   // This function will only be executed once, even if called multiple times in parallel.
 *   return singleExecution(
 *     () => {
 *       console.log(`Fetching user ${userId}...`);
 *       return api.fetch(`/users/${userId}`);
 *     },
 *     `user-${userId}` // A simple, descriptive key
 *   );
 * }
 *
 * Promise.all([fetchUser('123'), fetchUser('123')]); // "Fetching user 123..." is logged only once.
 *
 * // Example 2: Using an object as a key
 * async function searchProducts(filters: object) {
 *   return singleExecution(
 *     () => api.post('/products/search', filters),
 *     filters // The filters object is hashed to create a unique key
 *   );
 * }
 *
 * // Example 3: No key provided (hashes the function's source)
 * const fetchConfig = () => singleExecution(() => api.fetch('/config'));
 * Promise.all([fetchConfig(), fetchConfig()]); // The config is fetched only once.
 * ```
 */
export function singleExecution(taskFn, key) {
    return __awaiter(this, void 0, void 0, function* () {
        // If a key is provided, use it; otherwise, use the function's string representation.
        const keySource = key !== undefined ? key : taskFn.toString();
        const hashedKey = yield hashObject(keySource);
        const existingPromise = activeRequests.get(hashedKey);
        // If a request with the same key is already pending, return its promise.
        if (existingPromise) {
            return existingPromise;
        }
        // --- This is the first call for this key ---
        // Create the promise and store it in the map immediately to handle race conditions.
        const newPromise = taskFn();
        activeRequests.set(hashedKey, newPromise);
        try {
            // Await the task's completion.
            return yield newPromise;
        }
        finally {
            // IMPORTANT: Once the promise settles (resolves or rejects), immediately
            // remove it from the map. This ensures the next call re-executes the task.
            // We check if the promise in the map is still the one we created,
            // preventing a race condition where a new task started before this one finished.
            if (activeRequests.get(hashedKey) === newPromise) {
                activeRequests.delete(hashedKey);
            }
        }
    });
}