modern-javascript-patterns

What this skill does

# Modern JavaScript Patterns

Comprehensive guide for mastering modern JavaScript (ES6+) features, functional programming patterns, and best practices for writing clean, maintainable, and performant code.

## When to Use This Skill

- Refactoring legacy JavaScript to modern syntax
- Implementing functional programming patterns
- Optimizing JavaScript performance
- Writing maintainable and readable code
- Working with asynchronous operations
- Building modern web applications
- Migrating from callbacks to Promises/async-await
- Implementing data transformation pipelines

## ES6+ Core Features

### 1. Arrow Functions

**Syntax and Use Cases:**
```javascript
// Traditional function
function add(a, b) {
  return a + b;
}

// Arrow function
const add = (a, b) => a + b;

// Single parameter (parentheses optional)
const double = x => x * 2;

// No parameters
const getRandom = () => Math.random();

// Multiple statements (need curly braces)
const processUser = user => {
  const normalized = user.name.toLowerCase();
  return { ...user, name: normalized };
};

// Returning objects (wrap in parentheses)
const createUser = (name, age) => ({ name, age });
```

**Lexical 'this' Binding:**
```javascript
class Counter {
  constructor() {
    this.count = 0;
  }

  // Arrow function preserves 'this' context
  increment = () => {
    this.count++;
  };

  // Traditional function loses 'this' in callbacks
  incrementTraditional() {
    setTimeout(function() {
      this.count++;  // 'this' is undefined
    }, 1000);
  }

  // Arrow function maintains 'this'
  incrementArrow() {
    setTimeout(() => {
      this.count++;  // 'this' refers to Counter instance
    }, 1000);
  }
}
```

### 2. Destructuring

**Object Destructuring:**
```javascript
const user = {
  id: 1,
  name: 'John Doe',
  email: '[email protected]',
  address: {
    city: 'New York',
    country: 'USA'
  }
};

// Basic destructuring
const { name, email } = user;

// Rename variables
const { name: userName, email: userEmail } = user;

// Default values
const { age = 25 } = user;

// Nested destructuring
const { address: { city, country } } = user;

// Rest operator
const { id, ...userWithoutId } = user;

// Function parameters
function greet({ name, age = 18 }) {
  console.log(`Hello ${name}, you are ${age}`);
}
greet(user);
```

**Array Destructuring:**
```javascript
const numbers = [1, 2, 3, 4, 5];

// Basic destructuring
const [first, second] = numbers;

// Skip elements
const [, , third] = numbers;

// Rest operator
const [head, ...tail] = numbers;

// Swapping variables
let a = 1, b = 2;
[a, b] = [b, a];

// Function return values
function getCoordinates() {
  return [10, 20];
}
const [x, y] = getCoordinates();

// Default values
const [one, two, three = 0] = [1, 2];
```

### 3. Spread and Rest Operators

**Spread Operator:**
```javascript
// Array spreading
const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const combined = [...arr1, ...arr2];

// Object spreading
const defaults = { theme: 'dark', lang: 'en' };
const userPrefs = { theme: 'light' };
const settings = { ...defaults, ...userPrefs };

// Function arguments
const numbers = [1, 2, 3];
Math.max(...numbers);

// Copying arrays/objects (shallow copy)
const copy = [...arr1];
const objCopy = { ...user };

// Adding items immutably
const newArr = [...arr1, 4, 5];
const newObj = { ...user, age: 30 };
```

**Rest Parameters:**
```javascript
// Collect function arguments
function sum(...numbers) {
  return numbers.reduce((total, num) => total + num, 0);
}
sum(1, 2, 3, 4, 5);

// With regular parameters
function greet(greeting, ...names) {
  return `${greeting} ${names.join(', ')}`;
}
greet('Hello', 'John', 'Jane', 'Bob');

// Object rest
const { id, ...userData } = user;

// Array rest
const [first, ...rest] = [1, 2, 3, 4, 5];
```

### 4. Template Literals

```javascript
// Basic usage
const name = 'John';
const greeting = `Hello, ${name}!`;

// Multi-line strings
const html = `
  <div>
    <h1>${title}</h1>
    <p>${content}</p>
  </div>
`;

// Expression evaluation
const price = 19.99;
const total = `Total: $${(price * 1.2).toFixed(2)}`;

// Tagged template literals
function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => {
    const value = values[i] || '';
    return result + str + `<mark>${value}</mark>`;
  }, '');
}

const name = 'John';
const age = 30;
const html = highlight`Name: ${name}, Age: ${age}`;
// Output: "Name: <mark>John</mark>, Age: <mark>30</mark>"
```

### 5. Enhanced Object Literals

```javascript
const name = 'John';
const age = 30;

// Shorthand property names
const user = { name, age };

// Shorthand method names
const calculator = {
  add(a, b) {
    return a + b;
  },
  subtract(a, b) {
    return a - b;
  }
};

// Computed property names
const field = 'email';
const user = {
  name: 'John',
  [field]: '[email protected]',
  [`get${field.charAt(0).toUpperCase()}${field.slice(1)}`]() {
    return this[field];
  }
};

// Dynamic property creation
const createUser = (name, ...props) => {
  return props.reduce((user, [key, value]) => ({
    ...user,
    [key]: value
  }), { name });
};

const user = createUser('John', ['age', 30], ['email', '[email protected]']);
```

## Asynchronous Patterns

### 1. Promises

**Creating and Using Promises:**
```javascript
// Creating a promise
const fetchUser = (id) => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (id > 0) {
        resolve({ id, name: 'John' });
      } else {
        reject(new Error('Invalid ID'));
      }
    }, 1000);
  });
};

// Using promises
fetchUser(1)
  .then(user => console.log(user))
  .catch(error => console.error(error))
  .finally(() => console.log('Done'));

// Chaining promises
fetchUser(1)
  .then(user => fetchUserPosts(user.id))
  .then(posts => processPosts(posts))
  .then(result => console.log(result))
  .catch(error => console.error(error));
```

**Promise Combinators:**
```javascript
// Promise.all - Wait for all promises
const promises = [
  fetchUser(1),
  fetchUser(2),
  fetchUser(3)
];

Promise.all(promises)
  .then(users => console.log(users))
  .catch(error => console.error('At least one failed:', error));

// Promise.allSettled - Wait for all, regardless of outcome
Promise.allSettled(promises)
  .then(results => {
    results.forEach(result => {
      if (result.status === 'fulfilled') {
        console.log('Success:', result.value);
      } else {
        console.log('Error:', result.reason);
      }
    });
  });

// Promise.race - First to complete
Promise.race(promises)
  .then(winner => console.log('First:', winner))
  .catch(error => console.error(error));

// Promise.any - First to succeed
Promise.any(promises)
  .then(first => console.log('First success:', first))
  .catch(error => console.error('All failed:', error));
```

### 2. Async/Await

**Basic Usage:**
```javascript
// Async function always returns a Promise
async function fetchUser(id) {
  const response = await fetch(`/api/users/${id}`);
  const user = await response.json();
  return user;
}

// Error handling with try/catch
async function getUserData(id) {
  try {
    const user = await fetchUser(id);
    const posts = await fetchUserPosts(user.id);
    return { user, posts };
  } catch (error) {
    console.error('Error fetching data:', error);
    throw error;
  }
}

// Sequential vs Parallel execution
async function sequential() {
  const user1 = await fetchUser(1);  // Wait
  const user2 = await fetchUser(2);  // Then wait
  return [user1, user2];
}

async function parallel() {
  const [user1, user2] = await Promise.all([
    fetchUser(1),
    fetchUser(2)
  ]);
  return [user1, user2];
}
```

**Advanced Patterns:**
```javascript
// Async IIFE
(async () => {
  const result = await someAsyncOperation();
  console.log(result);
})();

// Async iteration
async function processUsers(userIds) {
  for (const id of userIds) {
    const user = await fetchUser(id);
    await processUser(user);
  }
}

// Top-level await (ES2