Generics

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Generics#

A generic is a type parameter. <T> declares the slot; the caller (or inference) fills it.

Generic function.

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

Inference at the call site.

const n = head([1, 2, 3]);            // n: number | undefined

const s = head(["a", "b"]);           // s: string | undefined

Generic class.

class Container<T> {
  constructor(public value: T) {}
}

Constraints narrow what T can be.

function pluck<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

The constraint catches a bad key at compile time.

pluck({name: "rk", age: 30}, "name");   // string
pluck({name: "rk", age: 30}, "age");    // number
pluck({name: "rk", age: 30}, "xyz");    // Error: not a key

References#

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