Classes and objects

A class is a blueprint describing state (fields) and behavior (methods); an object is a concrete instance with identity, lifecycle, and invariants enforced at construction. In JS/TS, classes are syntactic sugar over prototypes—`new User('Ada')` creates an instance with its own property storage linked to shared methods on the prototype chain.

Distinguish type (class/interface) from instance (object in memory). Identity matters when you track entities by ID, not merely by field values.

On interviews: explain constructor invariants, why two objects with equal fields are not the same entity, and how this maps to prototypes in JavaScript.

Common pitfalls: treating classes as namespaces of static functions, mutable singletons posing as objects, and confusing class references with instances.

The trade-off is modeling clarity versus boilerplate—use classes when lifecycle and identity are first-class.

Checklist:

• Define class versus instance versus value.
• Name invariants enforced in constructor.
• Relate JS class syntax to prototypes.