Generics Basics

Why generics exist

Without generics you often fall back to any. A type parameter keeps input and output related.

function first<T>(items: T[]): T | undefined {
  return items[0];
}

console.log(first([10, 20]));
console.log(first(["a", "b"]));

Constrain a type parameter

extends limits T to types that have the required shape.

function labelOf<T extends { name: string }>(item: T): string {
  return item.name;
}

console.log(labelOf({ name: "Course", id: 1 }));

Generic interfaces

Containers like boxes or API responses often take a type argument.

interface Box<T> {
  value: T;
}

const numberBox: Box<number> = { value: 42 };
const textBox: Box<string> = { value: "hi" };

Keep generics simple

• Start with one type parameter named T
• Add constraints only when you need specific properties
• Let inference work at call sites when possible
• Do not introduce generics when a concrete type is enough