Javascript

JavaScript

Modern JavaScript development with ES6+ features, async patterns, and best practices.

When to Use

• Working with .js files in web or Node.js projects
• Implementing async/await patterns and promises
• DOM manipulation and event handling
• Building frontend applications

Quick Start

// Modern async function with error handling
async function fetchData(url) {
  try {
    const response = await fetch(url);
    if (!response.ok) throw new Error(`HTTP ${response.status}`);
    return await response.json();
  } catch (error) {
    console.error("Fetch failed:", error);
    throw error;
  }
}

Core Concepts

Destructuring & Spread

// Object destructuring with defaults
const { name, age = 18, ...rest } = user;

// Array destructuring
const [first, second, ...remaining] = items;

// Spread for immutable updates
const updated = { ...user, name: "New Name" };
const combined = [...array1, ...array2];

Optional Chaining & Nullish Coalescing

// Safe property access
const city = user?.address?.city;
const firstItem = items?.[0];
const result = callback?.();

// Nullish coalescing (null/undefined only)
const value = input ?? defaultValue;

// Logical assignment
user.name ??= "Anonymous";
user.permissions ||= [];

Common Patterns

Async/Await Best Practices

Problem: Managing multiple async operations efficiently.

Solution:

// Parallel execution
async function fetchAllData(ids) {
  const promises = ids.map((id) => fetchData(id));
  return Promise.all(promises);
}

// Sequential with error handling
async function processItems(items) {
  const results = [];
  for (const item of items) {
    try {
      results.push(await processItem(item));
    } catch (error) {
      results.push({ error: error.message, item });
    }
  }
  return results;
}

// AbortController for cancellation
async function fetchWithTimeout(url, ms = 5000) {
  const controller = new AbortController();
  const timeout = setTimeout(() => controller.abort(), ms);

  try {
    return await fetch(url, { signal: controller.signal });
  } finally {
    clearTimeout(timeout);
  }
}

Modules

// Named exports (preferred)
export const API_URL = "https://api.example.com";
export function fetchUser(id) {
  /* ... */
}
export class UserService {
  /* ... */
}

// Re-exports
export { default as Button } from "./Button.js";
export * from "./utils.js";

// Dynamic imports
const module = await import(`./features/${feature}.js`);

Best Practices

Do:

• Use const by default, let when reassignment needed
• Use optional chaining (?.) for safe property access
• Use Promise.allSettled() for independent async operations
• Use named exports for better tree-shaking

Don't:

• Use var (prefer const/let)
• Use == for comparison (use ===)
• Nest callbacks deeply (use async/await)
• Mutate arrays/objects directly (use spread/map/filter)

Troubleshooting

Error	Cause	Solution
Cannot read property of undefined	Accessing nested property on null/undefined	Use optional chaining ?.
Uncaught (in promise)	Unhandled promise rejection	Add .catch() or try/catch
X is not a function	Calling undefined method	Check if method exists before calling

References

• MDN JavaScript Guide
• JavaScript.info