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.