JavaScript

Core JavaScript language patterns — runtime-agnostic, applies equally to browser and Node.js code.

Async

Async JavaScript Patterns: Promise combinators including allSettled, race timeouts, retry with backoff, and concurrency-limited mapping. Use when writing async control flow.
Async Pipe: Compose async functions into a sequential pipeline where each step receives the previous output. Use when chaining multiple async transformations.
Deferred Promise: A promise whose resolve/reject are accessible from outside the executor. Use when the creator and settler of a promise are in different scopes.
Delay and Interval Utilities: Promise-based delays, random stagger for animations, and frame-synced intervals. Use when you need timing utilities in async code.
Non-Concurrent Operations: Lock class and nonConcurrent wrapper that serialize async operations. Use when only one instance of an operation should run at a time.
Progress Tracking: EventEmitter-based progress reporting for batch async operations. Use when showing progress feedback for uploads, API calls, or parallel tasks.
Resolve All Promises: Prevent event loop hangs from unresolved promises in background async work. Use when running fire-and-forget async functions, background loops, or async iterator consumers.
Timer Patterns: Cancellable, awaitable timers with explicit state management and elapsed-time measurement. Use when scheduling one-shot actions or measuring durations.

Async Iterators with Repeater: Repeater library fundamentals — push/stop, backpressure, SlidingBuffer, merge, and latest combinators. Use when building async iterables.
Chunking Async Iterables: Fixed-size batching for arrays and async streams. Use when processing data in bulk.
Debouncing Async Iterables: Collapse rapid async events into single values after a quiet period. Use when coalescing bursts from event sources.
Event to Async Iterator: Convert EventEmitter or EventTarget sources into async iterables with automatic cleanup. Use when bridging events to for await...of.
Multicasting Async Iterables: Fan out values from one async iterable to multiple subscribers. Use when multiple consumers need the same stream.

Dot-Path Property Access: Tagged template literal that safely traverses deeply nested properties like user.profile.address.city. Use when reading nested object paths known at definition time.
Filter Object: Create a new object containing only allow-listed keys. Use when sanitizing user input or stripping unexpected properties.
Options with Defaults: Merge user options and default options using spread or nullish coalescing. Use when a function accepts optional configuration.

Index File Re-exports: Barrel files that re-export directory modules through a single entry point, converting defaults to named exports. Use when organizing module imports into clean paths.

Automatic Resource Management: Create-use-dispose lifecycle that guarantees resource cleanup even when errors occur. Use when managing temp dirs, connections, or file handles.
Composable Schema Validation: Chainable validators with message overrides, nesting, and value coercion. Use when validating user input, form fields, or API payloads.
Enum Classes: Sealed, frozen enum instances with identity semantics and iteration. Use when you need a fixed set of named values that prevent invalid states.
Layered Dispatch: First-match routing that dispatches items to handlers based on predicate tests, like Express middleware. Use when processing heterogeneous collections where each item needs different treatment.
Readable Regular Expressions: Tagged template literals that build regex from named, composable pieces. Use when a regex is too complex to read as a single literal.
RelativeURL: URL subclass that works with relative paths without requiring an origin. Use in SPAs or server-side route generation where the origin is irrelevant.
Result Type: Frozen Result class with ok/err factories and a try() wrapper that makes error handling explicit instead of throwing. Use for parsing, validation, or I/O where failure is expected.
Class-Based State Machines: EventEmitter-based state machine with lifecycle hooks and defined transitions. Use when a workflow has explicit states and boolean flags become unmanageable.

Random Utilities: Helper functions for random floats in a range, random integers, Fisher-Yates shuffle, and picking random elements. Use when you need randomness utilities beyond raw Math.random().

Unit-of-Work Telemetry: Accumulate context throughout a unit of work and emit one rich log entry when it completes. Extends to function-level tracing via AsyncLocalStorage. Use when implementing structured logging or tracing.
Custom Logger: Pino-based logger factory with auto-detected module names, Google Cloud severity mapping, and child loggers. Use when setting up structured logging for an application.
Browser Log Forwarding: Forward browser-side log entries and uncaught errors to a server endpoint so client and server logs appear in one stream. Use when building full-stack apps.

Name	`javascript`
Description	JavaScript language patterns: async/await, promises, iterators, collections, modules, design patterns, randomness, logging, telemetry. Automatically invoke when writing runtime-agnostic JavaScript.