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In JavaScript, a Factory controls how an object is created, while a Singleton makes one instance or state holder intentionally shared within a defined scope. Choose a Factory when the implementation may vary or callers should not need to know construction details; choose a Singleton only when shared identity or coordination is genuinely required. Neither pattern is a universal default: ordinary object creation and explicit dependency injection are often clearer.
At a glance
| Name | Best for | Key distinction | Typical scope | Checked |
|---|---|---|---|---|
| Factory | Choosing or hiding how an object is constructed | Can create many objects, including different implementations | Each call or caller-managed lifetime | Nov 2026 |
| Singleton | Sharing one coordinator, identity, or state holder | Reuses one instance within a named boundary | Module, window, application, or another explicit scope | Nov 2026 |
What is the difference between Singleton and Factory in JavaScript?
A Factory is a creation boundary; a Singleton is a reuse and sharing rule. They answer different questions: “How should this object be made?” versus “Should callers share the same object or state?”
A factory function can return a fresh object on every call, select one of several concrete implementations, or configure an object before returning it. A Singleton intentionally exposes one shared instance or state holder within a scope. A Factory can even create a Singleton, but the patterns are not interchangeable: one concerns construction, the other concerns identity and lifetime.
| Question | Factory | Singleton |
|---|---|---|
| Primary purpose | Hide or centralize construction choices | Coordinate intentionally shared state or identity |
| How many objects? | Often many; each call may create a new object | One reused object within the stated scope |
| How does the caller get it? | Calls a function or creation method, or receives one by injection | Uses an accessor or imports a module-owned object |
| What should be explicit? | The implementation choice or creation boundary | The boundary in which the instance is shared |
| Common trade-off | Another layer to maintain if construction is already simple | Implicit access and state that can complicate isolation |
MDN’s explanation of the new operator makes the ordinary baseline clear: “Each time you call new, a different instance is created.” A Factory may use new internally, but the caller does not have to select the concrete constructor. The W3C Design Principles likewise caution that “Factory methods can complement constructors, but generally should not be used instead of them.” A factory is useful when it hides a meaningful decision, not as ceremony around every new.
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What is a Factory pattern in JavaScript?
A Factory is a function or method that creates an object while keeping concrete construction details behind a named boundary. Callers request the capability or kind of object they need instead of assembling or selecting every implementation themselves.
For example, a notification feature could support email and SMS. The rest of the application can rely on both objects having a send method:
class EmailNotifier {
send(message) {
return `Email: ${message}`;
}
}
class SmsNotifier {
send(message) {
return `SMS: ${message}`;
}
}
function createNotifier(channel) {
switch (channel) {
case "email":
return new EmailNotifier();
case "sms":
return new SmsNotifier();
default:
throw new Error(`Unsupported channel: ${channel}`);
}
}
const notifier = createNotifier("email");
console.log(notifier.send("Your receipt is ready"));The useful boundary is createNotifier. A caller does not need to know which class to instantiate, and the supported choices and failure for an unsupported channel are visible in one place. If a new implementation is added, the creation decision can be updated there rather than scattered across call sites. The example still creates a new notifier whenever the factory is called; it does not imply that the returned object is shared.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIn a larger application, the factory might also accept configuration, choose an adapter based on runtime settings, or look up a registered implementation. Keep the choice proportionate: for a fixed, trivial object, a direct constructor or object literal is simpler. If the factory’s switch keeps growing, a registry can centralize mappings, but that registry is still a creation mechanism—not automatically a Singleton requirement.
A browser factory example
Factories need not be custom classes or elaborate frameworks. The browser’s document.createElement() API is a familiar factory-style interface: request an element by name and the browser returns the corresponding element object, handling its concrete construction details.
const button = document.createElement("button");
button.textContent = "Save";
const label = document.createElement("label");
label.textContent = "Email address";This demonstrates the pattern’s central idea, not a claim that every factory must follow a formal class hierarchy. In JavaScript, factories commonly appear as plain functions, static methods, registry lookups, or platform APIs.
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What is a Singleton pattern in JavaScript?
A Singleton provides one intentionally shared instance or state holder inside a defined scope. The phrase “one instance” is incomplete unless you say one per what: module instance, browser window, application, bundle, worker, or process.
For example, navigator, history, and crypto are browser-provided services available through a window or global environment. The W3C Design Principles discusses such objects as singleton-like platform objects. They illustrate scoped shared services; they do not mean every application service should become a global Singleton.
Likewise, a module can own shared state without exporting a class whose constructor tries to prevent all other instances:
// settings.js
const settings = new Map();
export function getSettings() {
return settings;
}// feature.js
import { getSettings } from "./settings.js";
const settings = getSettings();
settings.set("theme", "dark");Within a given loaded module instance, imports provide access to the module-owned map. MDN’s JavaScript modules documentation describes the import/export model that makes this approach possible. The scope qualification matters: duplicated bundles, separate realms, workers, or separate application processes may have separate module instances and therefore separate maps. This is not a cross-tab or process-wide storage guarantee.
For state that is only needed by one part of a feature, keeping it local or passing it to the components that need it is often easier to reason about than exporting a shared module object. Shared access is justified when multiple parts of the application must coordinate through the same identity or state, not merely because a convenient import is available.
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How do I create a Singleton with a JavaScript class?
JavaScript has private class fields but no native private constructor. A class can guard construction and expose an accessor, but the guard is an implementation convention rather than a language feature that makes every alternate construction path impossible.
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class AppConfig {
static #instance;
static getInstance() {
return (AppConfig.#instance ??= new AppConfig());
}
constructor() {
if (AppConfig.#instance) {
return AppConfig.#instance;
}
this.environment = "production";
}
}
const first = AppConfig.getInstance();
const second = AppConfig.getInstance();
console.log(first === second); // trueThe private static field #instance keeps the reference inside the class. The first call to getInstance() constructs an object and stores it; later calls return the stored reference. The constructor guard also makes direct calls to new AppConfig() return the existing object after initialization. MDN documents private class elements and private static elements; they help encapsulate this state, but JavaScript does not supply a dedicated private-constructor keyword.
That distinction affects how strongly the pattern is enforced. The class is intentionally designed to reuse its object, but it is not the same as a language-enforced prohibition on all construction. For many applications, a module-owned service or an object created once by an application container is more straightforward than adding a Singleton class solely to hide its constructor.
Do not use a Singleton as a shortcut for dependency injection. If a component reaches into a global accessor whenever it needs a service, its dependency is less visible and harder to replace. Passing the service in a constructor or function parameter makes that relationship explicit and allows the caller to control lifetime.
When should I use a Factory, and when should I use a Singleton?
Use a Factory when construction choices need a stable boundary. Use a Singleton only when one shared identity within a clearly named scope is part of the requirement.
- Choose a Factory when input or configuration determines the concrete type; you expect implementations such as transports, adapters, or plugins to vary; setup should be centralized; or tests need to supply a substitute through a visible creation boundary.
- Choose a Singleton when multiple parts of one defined scope must coordinate through the same object, such as a shared registry or cache, and the team accepts its lifetime and state-reset implications.
- Choose ordinary construction when the object is simple and the caller should own its lifetime. A direct
new SomeService(options)is often clearer than a factory or Singleton wrapper. - Choose dependency injection when a container or application composition root should create a service once and pass it to consumers. This can provide shared lifetime without making a hidden global accessor the only way to obtain it.
The key distinction is that a Factory does not have to return the same object twice, and a Singleton does not have to offer multiple implementation choices. If a requirement says “the right implementation depends on the environment,” start with a factory. If it says “these consumers must coordinate through this same state,” define the scope and then decide how to provide that shared state.
How do Factories and Singletons affect testing?
Factories usually make substitutions easier because the creation boundary is explicit: a caller can receive a fake factory or a test implementation. A Singleton can also be tested, but persistent shared state needs deliberate isolation so one test’s changes do not affect another.
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This is a practical design trade-off, not a claim that either pattern guarantees testability. For example, a factory can accept a dependency or be replaced at the composition boundary. A module-scoped Singleton may need a reset mechanism, a fresh module environment, or careful test setup when its state changes. Each reset mechanism adds lifecycle behavior that the application and tests must understand.
Prefer testing a consumer with a dependency passed in directly when possible. That makes its requirements visible and avoids coupling every test to a hidden global accessor. If a shared instance is necessary, keep its state small, document who owns it, and make any test isolation strategy explicit rather than relying on test order.
What mistakes should I avoid?
The most common mistake is treating either pattern as automatically beneficial. Add a creation boundary or shared instance only when it solves a concrete problem.
- Calling every factory a Singleton: a factory may return a fresh object each time. State its reuse behavior separately.
- Calling every module export globally unique: module sharing applies within a module instance, not automatically across duplicated bundles, realms, workers, windows, or processes.
- Using a Singleton for convenience alone: global reach can hide dependencies and make state changes harder to trace. Pass dependencies explicitly when that improves clarity.
- Replacing constructors everywhere with factories: W3C guidance says factories can complement constructors and generally should not replace them. Use a factory when the creation decision is useful to centralize.
- Ignoring lifetime: a shared cache or registry can persist longer than a component expects. Decide who initializes it, who can mutate it, and when it is reset or discarded.
- Assuming one instance means one per entire JavaScript ecosystem: browser windows, module instances, workers, and application processes have distinct boundaries. Name the intended boundary in design discussions and code documentation.
There is no performance figure that settles this choice. The relevant decision is architectural: whether construction needs abstraction, whether shared identity is required, and whether the resulting lifetime is understandable to callers and tests.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should I decide for a real feature?
Write down the behavior before selecting a pattern. A short design check often prevents an unnecessary abstraction:
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- Identify the required identity. Do consumers need one shared object, or would separate instances work just as well?
- Name the scope. If one shared object is needed, specify module, window, application, or another exact boundary.
- Choose who owns creation and lifetime. It may be a factory, the caller, or an application container. Avoid letting a global accessor silently decide both.
- Check the test seam. Can a consumer receive a substitute? If shared state persists, how will tests isolate or reset it?
- Keep the smallest solution. Start with direct construction if no meaningful variation or sharing requirement exists, then introduce a pattern when the requirement appears.
For a configurable notifier, the type-selection requirement points toward a Factory. For a registry that truly must be shared by features within one application module, module-owned state may be enough. For a stateless service used in one place, neither pattern may add value. The pattern follows the requirement, not the other way around.
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What is the practical rule to remember?
Factory controls how objects are created; Singleton controls whether a shared object is reused. Factories can make implementation choices explicit and substitutable. Singletons can coordinate shared state, but only within a clearly defined boundary and with lifecycle consequences that the design accepts. For many JavaScript applications, module-scoped services or explicit dependency injection are clearer than a globally reachable Singleton.
Frequently Asked Questions
Is a JavaScript factory function the same as a constructor?
No. A constructor is commonly called with new to initialize an instance, while a factory function can return an object without requiring new and can choose among different implementations. A factory may call constructors internally, so they can work together.
Does a Singleton in one browser tab also exist in another tab?
Not by default. A module or window-scoped object belongs to that loaded environment; separate tabs have separate JavaScript environments, so sharing across tabs requires a separate communication or storage mechanism.
Can a Factory return the same object every time?
Yes, a factory can return a cached or shared object, but doing so adds a reuse policy beyond the basic creation role. Document that behavior clearly so callers do not assume each factory call returns a fresh instance.
Does a Singleton make a JavaScript service faster?
There is no general performance guarantee from choosing the pattern. Choose it for required shared identity or coordination, and evaluate performance claims against a specific workload and credible measurements rather than assuming one instance is inherently faster.
