# Generic Interfaces, Classes & Constraints — TypeScript

Source: https://www.geekswithgeeks.com/en/typescript/ts-generic-classes-constraints

> Generic containers, and limiting T to types with certain properties.

## Generic interface & class

A generic class or interface reuses one definition for many payload types — a `Box<string>` and a `Box<number>` share the same shape.

```typescript
interface Box<T> {
  value: T;
}

class Container<T> {
  constructor(public value: T) {}
  get(): T {
    return this.value;
  }
}

const c = new Container<number>(42);
```

## Constraining with extends

`T extends { length: number }` limits `T` to types that at least have a `length` property — narrower than "anything", wide enough to be reusable.

```typescript
function logLength<T extends { length: number }>(item: T): void {
  console.log(item.length);
}

logLength("hello");     // OK, strings have length
logLength([1, 2, 3]);   // OK, arrays have length
logLength(42);
// Error: Argument of type 'number' is not assignable to parameter of type '{ length: number }'.
```

## Name generics meaningfully

`T` is fine for one generic. With several, prefer readable names — `TKey`, `TValue` — over `T`, `U`, `V`.