Generics

Generics let you write a function or type once and have it work correctly with whatever type gets passed in, without losing type safety.

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

console.log(firstElement([1, 2, 3]));        // inferred as number | undefined
console.log(firstElement(['a', 'b', 'c']));   // inferred as string | undefined

interface Box<T> {
  value: T;
}

const numberBox: Box<number> = { value: 42 };
console.log(numberBox.value);

Think of <T> as a placeholder: "whatever type you give me, I'll give the same type back out." The compiler figures out T automatically from the argument you pass, and still catches mistakes, firstElement([1, 2, 3])[0].toUpperCase() would be a compile error because that element is a number, not a string.