---

name: Writing JavaScript description: Writes modern JavaScript with ES2024+ features, async patterns, and functional programming. Use when building Node.js applications, implementing async workflows, or writing clean maintainable code. category: languages triggers:

• javascript
• js
• es6
• es2024
• ecmascript
• node
• nodejs
• npm

---

Writing JavaScript

Quick Start

Copy// Modern destructuring and spread
const { name, email, role = 'user' } = user;
const settings = { ...defaults, theme: 'dark' };
const [first, ...rest] = items;

// Optional chaining and nullish coalescing
const street = user?.address?.street ?? 'Unknown';
config.timeout ??= 5000;

// Async/await with error handling
async function fetchUser(id) {
  try {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) throw new Error(`HTTP ${response.status}`);
    return await response.json();
  } catch (error) {
    console.error('Failed to fetch:', error);
    throw error;
  }
}

// Functional composition
const pipe = (...fns) => x => fns.reduce((acc, fn) => fn(acc), x);
const processData = pipe(filter(isValid), map(transform), reduce(aggregate, []));

Features

Common Patterns

Async Patterns with Concurrency Control

Copy// Parallel execution
const [user, posts] = await Promise.all([fetchUser(id), fetchPosts(id)]);

// Handle mixed success/failure
const results = await Promise.allSettled(ids.map(fetchUser));
const successful = results.filter(r => r.status === 'fulfilled').map(r => r.value);
const failed = results.filter(r => r.status === 'rejected').map(r => r.reason);

// Retry with exponential backoff
async function withRetry(fn, maxRetries = 3, baseDelay = 1000) {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    try {
      return await fn();
    } catch (error) {
      if (attempt === maxRetries - 1) throw error;
      await new Promise(r => setTimeout(r, baseDelay * Math.pow(2, attempt)));
    }
  }
}

// Concurrent execution with limit
async function mapConcurrent(items, fn, limit = 5) {
  const results = [];
  const executing = new Set();
  for (const item of items) {
    const promise = fn(item).then(r => { executing.delete(promise); return r; });
    executing.add(promise);
    results.push(promise);
    if (executing.size >= limit) await Promise.race(executing);
  }
  return Promise.all(results);
}

Functional Programming Utilities

Copy// Currying
const curry = fn => function curried(...args) {
  return args.length >= fn.length ? fn(...args) : (...next) => curried(...args, ...next);
};

// Memoization
const memoize = fn => {
  const cache = new Map();
  return (...args) => {
    const key = JSON.stringify(args);
    if (!cache.has(key)) cache.set(key, fn(...args));
    return cache.get(key);
  };
};

// Immutable updates
const updateUser = (user, updates) => ({ ...user, ...updates, updatedAt: new Date() });
const addItem = (arr, item) => [...arr, item];
const removeAt = (arr, i) => [...arr.slice(0, i), ...arr.slice(i + 1)];

Error Handling with Result Type

Copyclass Result {
  constructor(value, error) { this.value = value; this.error = error; }
  static ok(value) { return new Result(value, null); }
  static err(error) { return new Result(null, error); }
  isOk() { return this.error === null; }
  map(fn) { return this.isOk() ? Result.ok(fn(this.value)) : this; }
  unwrapOr(defaultValue) { return this.isOk() ? this.value : defaultValue; }
}

async function safeFetch(url) {
  try {
    const res = await fetch(url);
    if (!res.ok) return Result.err(new Error(`HTTP ${res.status}`));
    return Result.ok(await res.json());
  } catch (e) { return Result.err(e); }
}

// Usage
const result = await safeFetch('/api/data');
const data = result.map(d => d.items).unwrapOr([]);

Event Emitter Pattern

Copyclass EventEmitter {
  constructor() { this.events = new Map(); }

  on(event, listener) {
    if (!this.events.has(event)) this.events.set(event, new Set());
    this.events.get(event).add(listener);
    return () => this.off(event, listener);
  }

  off(event, listener) { this.events.get(event)?.delete(listener); }

  emit(event, ...args) {
    this.events.get(event)?.forEach(listener => listener(...args));
  }

  once(event, listener) {
    const wrapper = (...args) => { listener(...args); this.off(event, wrapper); };
    return this.on(event, wrapper);
  }
}

Modern Collection Usage

Copy// Map for key-value with any type keys
const cache = new Map();
cache.set(userObj, 'data'); // Object as key
for (const [key, value] of cache) { /* iterate */ }

// Set for unique values and operations
const setA = new Set([1, 2, 3]);
const setB = new Set([2, 3, 4]);
const union = new Set([...setA, ...setB]);
const intersection = new Set([...setA].filter(x => setB.has(x)));

// WeakMap for private data without memory leaks
const privateData = new WeakMap();
class Secret {
  constructor(value) { privateData.set(this, { value }); }
  getValue() { return privateData.get(this).value; }
}

Best Practices

Related Skills

• typescript - Type-safe JavaScript development
• nodejs - Server-side JavaScript runtime
• react - JavaScript UI framework

References

• MDN JavaScript Guide
• ECMAScript Specification
• JavaScript Info
• Node.js Documentation