Patterns #3 — Singleton, Factory & Constructor

Singleton ensures one instance; Factory abstracts object creation; Constructor vs Factory is an architectural choice. This article compares all three with runnable examples and explains when each makes sense in JavaScript.

Prerequisites: JS Foundations #3 — Closures

1. Singleton — One Instance, Guaranteed

Closure-based singleton:

2. Factory Function — Create Objects Without new

Factory functions hide creation complexity and can return different implementations:

3. Constructor Function vs Factory Function

4. When to Use Which

Key Takeaways

• Singleton: one instance — ESM modules are natural singletons; closures can enforce it.
• Factory: creates objects without new — can return different shapes, hide implementation, use true private state.
• Constructor: needs new — has prototype chain, instanceof, shared methods (memory efficient).
• In modern JS, factories + closures often replace constructors — composition over inheritance.
• Use the pattern that fits the problem, not the pattern you're most comfortable with.

Next: Data Structures #1 — Linked List — implement singly and doubly linked lists with all operations.