Inverse TypeScript Type Guard

Question

I am using typescript 2.0.0 with --strictNullChecks and the following type guard:

function isNotOk(value: any): value is null | undefined {
  if (typeof value === 'number') {
    return !isFinite(value);
  } else {
    return value === null || value === undefined;
  }
}

Which invalidates null, undefined, NaN and Infinite. I want an inverse of this:

export function isOk(value: any): value is not null | undefined {
  return !isNotOk(value);
}

Of course, this syntax does not work. Is there a known way to accomplish this?

Answer 1

I stumbled upon the answer; generics. Just narrow in a reverse manner as such:

function isOk<T>(value: T | null | undefined): value is T {
  return !isNotOk(value);
}