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Types · easy

17. Generics basics

Writing reusable code that stays type-safe.

1 min read3 questionsup to 50 XPtopic 17/50

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Generics let a function, interface, or class work over a variety of types while preserving the relationship between input and output types.

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

first([1, 2, 3]);        // inferred T = number, returns number | undefined
first(["a", "b"]);       // inferred T = string

interface Box<T> {
  value: T;
}
const b: Box<string> = { value: "hi" };

The type parameter `T` is a placeholder filled in at each call site, either explicitly (`first<number>([1,2,3])`) or inferred from the arguments. Generics avoid the loss of type information that `any` would introduce while still allowing one implementation to serve many types.

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Check your understanding

  1. 1. What problem do generics solve?

  2. 2. In `first([1, 2, 3])`, how is `T` determined?

  3. 3. What does `Box<string>` mean for `interface Box<T> { value: T }`?

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