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Design Patterns

23 Gang of Four patterns · Creational, structural, behavioral · Node.js examples

01Creational Patterns- How objects are created without hard-coding construction everywhere

Singletoncreational

Ensure only one shared instance exists for a resource.

getInstance() -> exists? return same -> missing? create once

e.g.: Mongoose connection, Winston logger, config manager

Factory Methodcreational

Create objects through a factory instead of calling constructors directly.

create(type) email -> EmailNotifier sms -> SMSNotifier push -> PushNotifier

e.g.: Express middleware factories, Sequelize dialects

Abstract Factorycreational

Create families of related objects behind one common interface.

DBFactory -> MySQL connection + query -> Mongo connection + query

e.g.: TypeORM, Sequelize dialect system, cloud provider SDK wrappers

Buildercreational

Build complex objects step by step with a fluent API.

builder.select() -> from() -> where() -> limit() -> build()

e.g.: Knex.js, Mongoose query API, request builders

Prototypecreational

Clone an existing object and customize the copy.

base object -> clone() -> override fields -> new variant

e.g.: Object.create(), structuredClone(), config cloning

02Structural Patterns- How objects and modules are composed into larger shapes

Adapterstructural

Translate one interface into another interface your app expects.

Your code expects send(msg) -> Adapter -> SDK uses deliver({ body })

e.g.: AWS SDK wrappers, Twilio adapters, payment adapters

Bridgestructural

Separate abstraction from implementation so both can vary independently.

Report -> Renderer interface -> PDFRenderer / HTMLRenderer

e.g.: Renderer plugins, storage drivers, notification channels

Compositestructural

Treat single objects and groups of objects the same way.

Folder -> File -> Folder -> File call size() on any node

e.g.: File trees, React component trees, menu trees

Decoratorstructural

Wrap an object to add behavior without changing the original class.

Base service -> Auth wrapper -> Log wrapper -> Cache wrapper

e.g.: Express middleware, NestJS decorators, service wrappers

Facadestructural

Expose one simple API over a complex subsystem.

placeOrder() -> reserve inventory -> charge payment -> send email

e.g.: Express app.listen(), Mongoose.connect(), SDK clients

Flyweightstructural

Share repeated immutable data to reduce memory usage.

many objects -> shared intrinsic state -> per-item external state

e.g.: Object pools, string interning, cached metadata

Proxystructural

Use a stand-in object to control access to the real object.

Client -> Proxy checks auth/cache/rate -> Real service

e.g.: JavaScript Proxy, ORM lazy loading, caching proxy

03Behavioral Patterns- How objects communicate, delegate work, and manage behavior

Chain of Responsibilitybehavioral

Pass a request through handlers until one handles it.

Request -> auth -> validation -> controller -> response

e.g.: Express middleware, Koa middleware, error pipelines

Commandbehavioral

Wrap an action as an object so it can be queued, logged, or undone.

Client creates command -> queue/log/execute -> optional undo

e.g.: BullMQ jobs, task queues, undo stacks

Interpreterbehavioral

Represent grammar rules and evaluate expressions.

Input expression -> parse rules -> evaluate AST

e.g.: Template engines, rule engines, query parsers

Iteratorbehavioral

Traverse a collection without exposing how it is stored.

Collection -> iterator.next() -> item by item

e.g.: for...of, async iterators, streams

Mediatorbehavioral

Centralize communication so objects do not talk directly to each other.

Component A -> Mediator -> Component B/C

e.g.: Event bus, message broker wrappers, NestJS CQRS bus

Mementobehavioral

Capture and restore object state without exposing internals.

state now -> save snapshot -> changes -> restore snapshot

e.g.: Undo history, drafts, editor snapshots

Observerbehavioral

Notify subscribed listeners when state changes.

Subject emits event -> listener A -> listener B -> listener C

e.g.: EventEmitter, RxJS, DOM events, Redis pub/sub

Statebehavioral

Change behavior when an object changes internal state.

Order: pending -> paid -> shipped -> delivered

e.g.: Workflow engines, XState, order lifecycles

Strategybehavioral

Swap algorithms at runtime without changing the caller.

Context -> strategy.pay() -> Stripe / PayPal / Razorpay

e.g.: Passport strategies, payment strategies, sorting strategies

Template Methodbehavioral

Define an algorithm skeleton and let subclasses fill in steps.

generate() -> fetch -> format -> render steps vary

e.g.: Report generators, test runners, ETL jobs

Visitorbehavioral

Add operations to object structures without changing those objects.

Visitor -> visitUser() -> visitOrder() -> visitInvoice()

e.g.: AST transforms, Babel plugins, tree walkers

Design patterns are code-structure patterns. They help objects and modules stay flexible, but they should be used when they simplify real change, not just to make code look formal.