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JavaScript design patterns help when they reduce a specific kind of change, coupling, or construction complexity—not because applying more patterns automatically makes an application scalable. For most new projects, start with native JavaScript modules, then add a factory, observer, singleton, proxy, or mediator only when the problem it addresses is present.
What JavaScript design patterns solve
A design pattern is a reusable approach to a recurring software-design problem. It describes an idea, not a required code shape: the same idea can be implemented differently depending on the language and application. For a growing JavaScript application, the practical question is which parts of the code change together, which consumers know too much about an implementation, and where construction or coordination has become difficult.
JavaScript’s standardized core language is ECMAScript. Browser APIs and other host capabilities sit around the language rather than being part of the ECMAScript specification. That distinction matters because some mechanisms commonly described as patterns—especially modules—are now supported directly by the language. MDN’s JavaScript Guide covers imports, exports, and dynamic loading; its JavaScript technologies overview explains the distinction between ECMAScript and host APIs.
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Pick a pattern by the pressure in the code, not by a checklist. Compare the options against these practical questions:
#1 Best Overall
- Coupling: Do consumers need to know concrete implementation details?
- Change isolation: If behavior changes, how many callers need edits?
- Complexity: Does the added abstraction make the flow clearer, or merely add indirection?
- Testing and isolation: Can a test create a fresh instance without inheriting unrelated state?
- Lifecycle: Who owns subscriptions, resources, and cleanup?
- Existing mechanisms: Does JavaScript or the framework already provide the capability?
These are decision criteria, not a measured ranking of patterns. A small direct function or object is often easier to maintain than a named abstraction when there is only one caller or no meaningful variation.
Use native modules to organize code
When modules help
Modules are a natural starting point when related behavior should be grouped behind a deliberate public interface. An ES module can export the operations other parts of the application need while keeping implementation details private to the file. This supports change isolation without requiring a global namespace or a manually managed closure.
// price.js
const taxRate = 0.08;
export function priceWithTax(amount) {
return amount * (1 + taxRate);
}
// checkout.js
import { priceWithTax } from "./price.js";
const total = priceWithTax(25);How this differs from the historical Module pattern
The older closure-based Module pattern uses a function scope to hide state and return a public object. It can still be useful when explaining closures or encapsulating a small standalone script, but it is not the default way to organize a modern JavaScript application: native import and export already provide module boundaries. Use the closure idiom when its specific behavior is useful, not simply because “Module pattern” appears in a pattern catalog.
Rank #2
Use a factory when construction varies
The pressure it addresses
A factory is useful when callers should request a product without knowing every construction detail, or when the product depends on an input or environment. It can keep branching and setup out of callers. Patterns.dev describes Factory as a way to create objects: Factory pattern.
function createNotifier(channel) {
if (channel === "email") {
return { send: (message) => sendEmail(message) };
}
if (channel === "console") {
return { send: (message) => console.info(message) };
}
throw new Error(`Unsupported channel: ${channel}`);
}
const notifier = createNotifier("console");
notifier.send("Build complete");Here, callers depend on the returned send capability rather than the construction branch. The tradeoff is another layer to navigate and maintain. If there is only one straightforward implementation, constructing the object directly is simpler. A factory also should not conceal meaningful differences that callers need to handle.
Use Observer for one-to-many notifications
When subscriptions fit
Observer-style subscriptions are appropriate when one event or state change should notify multiple interested consumers without the producer directly depending on each consumer. Patterns.dev describes observables as notifying subscribers when an event occurs: Observer pattern.
function createEmitter() {
const listeners = new Set();
return {
subscribe(listener) {
listeners.add(listener);
return () => listeners.delete(listener);
},
emit(value) {
for (const listener of [...listeners]) {
try {
listener(value);
} catch (error) {
queueMicrotask(() => { throw error; });
}
}
}
};
}
const updates = createEmitter();
const unsubscribe = updates.subscribe((value) => render(value));
updates.emit({ status: "ready" });
unsubscribe();The returned cleanup function makes subscription ownership explicit. This example notifies listeners synchronously in insertion order, based on a snapshot of the listener set; a listener added during a notification will not receive that notification, and one removed during it may still be present in the snapshot. Listener failures are rethrown asynchronously rather than stopping the remaining listeners. Those are design choices, not universal Observer rules, so define the behavior your application needs.
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Treat Singleton as shared-state access, not a default
A Singleton provides one shared instance through a common access point. It can suit a resource that genuinely must be shared, but it couples consumers to shared state and can make tests, parallel work, and multiple application instances harder to isolate. Patterns.dev includes Singleton among its JavaScript pattern topics: Singleton pattern.
Rank #4
Before making a service global, consider passing it into the functions or components that need it. Explicit dependencies make it easier to replace the service in tests or create separate instances for different application contexts. Use shared-instance access only when the shared lifecycle is intentional and callers should not own separate instances.
Reserve Proxy, Mediator, and Middleware for concrete needs
Proxy for controlled access
A Proxy can intercept operations on another object, which is useful when access itself needs behavior such as validation or logging. Interception can also obscure what a property read or write actually does. Add it only when controlling access is a real requirement, not as a general wrapper around ordinary objects.
Mediator for coordination
A Mediator centralizes communication among components that would otherwise know about one another. That can reduce direct dependencies, but it may create a central object that accumulates unrelated rules. Consider it when coordination is genuinely distributed across multiple participants; for a simple interaction, a direct call is easier to follow.
Middleware for composable processing
Middleware composes processing stages, often around requests or other operations. It helps when stages need to be added, reordered, or reused independently. It also adds control-flow indirection: developers must understand how each stage passes work along and what happens when a stage stops or fails. Patterns.dev’s catalog covers these and other patterns across vanilla JavaScript and modern frameworks: Patterns.dev.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical progression for a growing application
- Separate responsibilities with native modules. Keep related behavior together and export only what other modules need.
- Keep straightforward construction direct. Introduce a factory when callers should be shielded from real construction variation.
- Use direct calls for direct relationships. Add Observer when multiple consumers need notifications without the producer knowing them individually.
- Make lifecycle and state ownership explicit. Prefer injected dependencies and clear cleanup over implicit global access where isolation matters.
- Add interception or coordination only for demonstrated pressure. Proxy, Mediator, and Middleware are tools for particular access, collaboration, and processing problems—not baseline application structure.
This progression is a way to decide, not a mandate to adopt every pattern. A pattern earns its place when the reduced coupling or isolated change is worth the complexity it introduces.
Where to learn more
MDN’s JavaScript Guide is a current reference for language features including modules, closures, classes, and promises. For a pattern-focused catalog that includes vanilla JavaScript and framework topics, see Patterns.dev. Addy Osmani’s Learning JavaScript Design Patterns is a book-length resource listed by Google Books; check the edition and compare examples with current language documentation before applying them to a modern codebase.
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