TypeScript Abstract Classes vs Interfaces: How to Choose the Right Tool in 2026

TypeScript developers often misuse abstract classes and interfaces by treating them as interchangeable, leading to duplicated logic and brittle codebases. Interfaces define pure structural contracts with no runtime footprint, making them ideal when multiple unrelated classes need to share a shape without shared code. Abstract classes combine contracts with executable shared behavior and state, reducing duplication when subclasses require common implementation logic. The core decision rule is straightforward: if the contract requires shared code, use an abstract class; if not, use an interface. Hybrid patterns that combine both — abstract classes implementing interfaces — can offer maximum flexibility by separating contract definition from base implementation.