How to filter out nullish values from an array in TypeScript (and get the right type)

When working with arrays in TypeScript, you might want to filter out null or undefined values from an array. Frustratingly, something like this doesn’t work as expected:

const values = [1, null, 2, undefined, 3];
const filtered = values.filter(
  (value) => value !== null && value !== undefined,
); // Type is (number | null | undefined)[]

This drives me mad and results in doing some weird type assertions to get the right type. After hitting this again today, I found a useful comment on one of the various issue threads related to this topic (source) which provides a cleaner solution:

values.filter((x): x is NonNullable<typeof x> => Boolean(x));

It’s still not perfect, and doesn’t help if you have a more complex type (e.g. you are checking if x.nestedKey is null), but it is cleaner than some of the garbage I have come up with before.