Generic functions, classes, constraints with extends, infer keyword.

TypeScript Generics — Type Parameters, Constraints, Inference

Generics allow TypeScript code to work with multiple types while preserving type safety. A type parameter <T> acts as a placeholder filled at call site or inferred by the compiler. Constraints (T extends ...) restrict what types a generic accepts. The infer keyword extracts types from complex type expressions inside conditional types. Generic inference eliminates the need to manually specify type arguments in most cases.

Interactive Pipeline Flow

UnderstandImplementOptimise

🔷 Type Param — function<T>

Type parameter: function identity<T>(arg: T): T { return arg; }. T is inferred from the argument — identity(42) returns number, identity("hi") returns string.

Architecture Diagrams

Using infer to extract types from complex type expressions.

Loading diagram...

Key Design Decisions

🔒Use constraints to make generics useful

An unconstrained <T> can only call methods that exist on all types (essentially nothing useful). T extends { id: number } lets you access .id. Constraints make generics practical.

🧠Prefer inference over explicit type arguments

TypeScript infers generic parameters from call-site arguments in almost all cases. Writing identity<string>("hello") instead of identity("hello") is noise. Inference is the default — use explicit only when needed.

🔍infer is the key to utility type patterns

The built-in ReturnType, Parameters, InstanceType, and Awaited utilities all use infer. Understanding infer unlocks the ability to write any type-level transformation.

Discussion