Restrict generic type T to not being undefined in TypeScript?

You can do it with NonNullable<T>, but not as a constraint on the type-parameters, but using it as an interface-type on get‘s R, like so:

function get<K, R>( o: { get: (key: K) => NonNullable<R> }, key: K ): NonNullable<R> {
  return o.get(key);
}

So this code gives me an error:

const params = new Params({ a: 10 });

console.log(get(params, "a"))

Argument of type ‘Params‘ is not assignable to parameter of type ‘{ get: (key: "a") => number; }‘.
The types returned by ‘get(…)’ are incompatible between these types.
Type ‘number | undefined‘ is not assignable to type ‘number‘.
Type ‘undefined‘ is not assignable to type ‘number‘.(2345)

The catch is that NonNullable<T> prohibits both undefined and null – which may or may not be desirable in your application.

Update

I found the definition of NonNullable<T> in lib.es6.d.ts:

/**
 * Exclude null and undefined from T
 */
type NonNullable<T> = T extends null | undefined ? never : T;

This can be tweaked to restrict only undefined:

type NonUndefined<T> = T extends undefined ? never : T;

and if I change get to this:

type NonUndefined<T> = T extends undefined ? never : T;

function get<K, R>(o: { get: (key: K) => NonUndefined<R> }, key: K): NonUndefined<R> {
  return o.get(key);
}

Then this works exactly as you’ve requested: it allows class Params‘s get to return number | null but not number | undefined.