/javascript-language

name: javascript-language
description: Modern JavaScript (ES2022+) patterns for clean, maintainable code. Use when working with modern JavaScript features like optional chaining, nullish coalescing, or ESM.
metadata:
  triggers:
    files:
    - '**/*.js'
    - '**/*.mjs'
    - '**/*.cjs'
    keywords:
    - const
    - let
    - arrow
    - async
    - await
    - promise
    - destructuring
    - spread
    - class

JavaScript Language Patterns

**Priority: P0 (CRITICAL)**

Implementation Guidelines

• **Variables**: const default. let if needed. No var — block scope only.
• **Functionality**: Use **`Arrow Functions`** for callbacks/inlines; **`Function Declaration`** for core logic.
• **Async Logic**: Use async/await with try/catch and Promise.all() for parallel operations. ESM import/export only. No Callbacks — promisify everything.
• **Modern Syntax**: Use Destructuring, Spread (...), Optional Chain ?. and Nullish ?? coalescing.
• **String Handling**: Use **`Template Literals`** (backticks) for interpolation and multi-line strings.
• **Data Collections**: Prefer **`map`**, **`filter`**, and **`reduce`** over imperative `for` loops.
• **Modules**: Standardize on ESM import/export; use Named Exports over Default.
• **Encapsulation**: Leverage **`#private`** class fields for encapsulation.
• **Error States**: Throw **`new Error()`** with descriptive messages; never throw strings.

Anti-Patterns

• No var — Block scope only.
• **No `==`**: Strict `===`.
• **No `new Object()`**: Use literals `{}`.
• No Callbacks: Promisify everything.
• **No Mutation**: Immutability first.

Code & Reference

See [references/REFERENCE.md](references/REFERENCE.md) for modern syntax, async patterns, private fields, and functional programming examples.

Related Topics

best-practices | tooling

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• ESM import/export
• No Callbacks
• No var
• Nullish ??
• Optional Chain ?.
• async/await
• const default
• let if needed
• user?.profile?.name

• Additional task-grounded exact anchors: try/catch