modern-javascript-patterns

SKILL.md

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:

// 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:

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:

const user = {
  id: 1,
  name: "John Doe",
  email: "john@example.com",
  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:

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:

// 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:

// 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

// 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

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]: "john@example.com",
  [`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", "john@example.com"]);

Asynchronous Patterns

1. Promises

Creating and Using Promises:

// 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:

// 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:

// 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:

// 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 (ES2022)
const config = await fetch("/config.json").then((r) => r.json());

// Retry logic
async function fetchWithRetry(url, retries = 3) {
  for (let i = 0; i < retries; i++) {
    try {
      return await fetch(url);
    } catch (error) {
      if (i === retries - 1) throw error;
      await new Promise((resolve) => setTimeout(resolve, 1000 * (i + 1)));
    }
  }
}

// Timeout wrapper
async function withTimeout(promise, ms) {
  const timeout = new Promise((_, reject) =>
    setTimeout(() => reject(new Error("Timeout")), ms),
  );
  return Promise.race([promise, timeout]);
}

Functional Programming Patterns

Functional programming in JavaScript centers on pure functions, immutability, and composable transformations.

Key topics covered in references/advanced-patterns.md:

Array methods — map, filter, reduce, find, findIndex, some, every, flatMap, Array.from

Higher-order functions — custom forEach/map/filter, currying, partial application, memoization

Composition and piping — compose/pipe utilities with practical data transformation examples

Pure functions and immutability — immutable array/object operations, deep cloning with structuredClone

Modern Class Features

ES2022 classes support private fields (#field), static fields, getters/setters, and private methods. See references/advanced-patterns.md for a full example with inheritance.

Modules (ES6)

// Named exports
export const PI = 3.14159;
export function add(a, b) { return a + b; }

// Default export
export default function multiply(a, b) { return a * b; }

// Import
import multiply, { PI, add } from "./math.js";

// Dynamic import (code splitting)
const { add } = await import("./math.js");

For re-exports, namespace imports, and conditional dynamic loading see references/advanced-patterns.md.

Iterators and Generators

Generators (function*) and async generators (async function*) enable lazy sequences and async pagination. See references/advanced-patterns.md for custom iterator, range generator, fibonacci, and for await...of examples.

Modern Operators

// Optional chaining — safe property access
const city = user?.address?.city;
const result = obj.method?.();

// Nullish coalescing — default only for null/undefined (not 0 or "")
const value = null ?? "default"; // 'default'
const zero = 0 ?? "default";    // 0

// Logical assignment
a ??= "default";   // assign if null/undefined
obj.count ||= 1;   // assign if falsy
obj.count &&= 2;   // assign if truthy

Performance Optimization

See references/advanced-patterns.md for debounce, throttle, and lazy evaluation with generators.

Best Practices

Use const by default: Only use let when reassignment is needed

Prefer arrow functions: Especially for callbacks

Use template literals: Instead of string concatenation

Destructure objects and arrays: For cleaner code

Use async/await: Instead of Promise chains

Avoid mutating data: Use spread operator and array methods

Use optional chaining: Prevent "Cannot read property of undefined"

Use nullish coalescing: For default values

Prefer array methods: Over traditional loops

Use modules: For better code organization

Write pure functions: Easier to test and reason about

Use meaningful variable names: Self-documenting code

Keep functions small: Single responsibility principle

Handle errors properly: Use try/catch with async/await

Use strict mode: 'use strict' for better error catching

For common pitfalls (this binding, promise anti-patterns, memory leaks), see references/advanced-patterns.md.